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A Guide to SIP Trunking

June 20, 2018

11:15 am

Session Initiation Protocol or SIP Trunking as it is commonly known is very similar to your conventional business phone line, but it relies on a virtual connection to a Public Switched Telephone Network (PSTN) making use of your internet connection.

Trunking is a term that was used in the early days of the telephone service. A trunk was a way in which large numbers of users could share a communications path. SIP today is just one of the protocols that make up the VoIP system that is becoming a business standard and enable them to function properly.

This guide will cover a number of areas related to SIP trunking including examining exactly what it is and how it works, the best SIP providers to choose for your business and also a SIP price comparison so you ensure you’re getting the best solutions for your budget.

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SIP Trunk Pricing Comparison

The number of SIP trunks your business needs is primarily determined by 2 factors: call volume and Internet connection.

Industry guidelines dictate that you’ll need roughly 1 SIP trunk for every 2-3 users. This will ensure that your system can support concurrent calls at peak times.

A strong, reliable Internet connection also plays an important role. Because SIP trunks use bandwidth, you may need to upgrade your Internet package if you think you’ll experience regular, heavy call volume (though this isn’t always necessary).

As for pricing, SIP trunks vary per provider. Some carriers bill per trunk with an additional cost per minute, while others charge a flat fee for each trunk and include unlimited usage.

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Vonage Logo$35.994.5 starsNo Contact Necessaryget-quote-button-green

SIP providers operate by supplying businesses with a trunk that links your PBX to the PSTN. It replaces the need for a traditional telephone trunk, and lets you streamline your telecom services.

SIP trunks combine data, voice and video in one line, eliminating the need for individual physical lines and media.  While all SIP trunking performs the same function, some providers connect the IP PBX virtually. Hosted or cloud-based carriers offer built-in PSTN connections, meaning your business doesn’t have to purchase trunks separately.

Rather, the provider hosts all of the services on its own servers, and allows you to use them.

While some providers offer free SIP trunking, the quality of these services is inconsistent. Businesses that require a reliable IP voice experience should instead consider one of the following providers.

Support 98%
Features 80%
Cost 95%
  • Reliable Support
  • Fraud Protection
  • Simple Pricing Structures

SIP.US puts the user in control of its SIP trunk services for IP PBX systems and analog-digital telephone adapters. Rather than locking customers in to a corresponding bandwidth plan, SIP lets you use your preferred bandwidth supplier.

Additionally, the company provides an easy-to-use, self-managed interface where you can add or change service, as well as review call data records. For a trunk with unlimited capacity, SIP.US charges $24.95 per channel (i.e., per line). Its phone number pricing amounts to $1.00 per month in the US 48, with international numbers starting at $5.00 per number / per month.

SIP.US is easy to manage and offers excellent customer service.

Support 90%
Features 95%
Cost 85%
  • Full Control
  • Customer Call Plans
  • Additional Phone

AT&T’s IP Flex Reach service is a technological step up from some of its competitors. In addition to basic SIP trunking, it uses voice compression, which delivers more traffic on the same circuit without compromising call quality. Centralized Call Delivery routes calls that originate from all over the country, and allows them to be answered at your preferred, central location.

The service has 3 calling plans (A, B and C); each one progressively offers more support for heavy long-distance and international rates. IP Flex Reach is ideal for medium to large businesses with heavy call volume and multiple locations.

8x8 Logo
Support 95%
Features 75%
Cost 80%
  • Keep Existing Phones
  • Inbound/Outbound Service
  • Toll-Free

8×8’s SIP trunk uses a VoIP dial tone solution that can be scaled to the needs of businesses large and small. The service provides free calling between 8×8-enabled systems, and emphasizes its pay-as-you-grow scalability.It also offers a variety of service plans, all based on 6-second billing increments.

As a leading name in digital communications and with 7,000 rate centers nation-wide, 8×8’s SIP trunking is reputable, and best suited for companies who may experience fluctuations in growth.

Support 80%
Features 95%
Cost 80%
  • Cut Costs
  • PBX Integration
  • No Activation

Net2Phone’s SIP trunking service allows businesses to take advantage of international DIDs (phone numbers) from over 50 countries.

Meanwhile, it supports unlimited concurrent calls, and offers a no-strings-attached pricing plan. The company offers free use of channels and no monthly contracts.

Net2Phone’s SIP trunking works well for businesses that have a heavy international call volume, and require the freedom of scalability.

Support 95%
Features 90%
Cost 92%
  • No Contracts
  • Excellent Support
  • Free Trial

Hosted through PBX, this is a business VoIP provider offering both affordable and reliable solutions for businesses. Ideal for companies who employ 50 staff or fewer, the business is a leading provider of VoIP solutions for all types of business. What’s great about Vonage is that they do own their own software which is a plus over some of the other providers and because of this they can offer a higher level of quality with their SIP services.

Pricing is charged on a monthly basis but customers aren’t locked in to long term contracts which always appeals to the budget conscious business. This SIP provider offers a number of advanced features with an easy to navigate interface and first class customer support and dedicated customer service when you need it.

Nextiva Logo
Support 92%
Features 99%
Cost 99%
  • Unlimited Calls
  • Use Existing Structure
  • Simple Setup

Nextiva offers businesses a range of SIP trunking options which are designed to save money on business communications. The systems also increase return on investment and allow businesses to take advantage of unlimited local and long distance calling features without changing existing PBX equipment.

What’s more, with Nextiva you can quickly and easily convert any traditional phone system into a dedicated IP based system to manage all of your communications. The company also offers comprehensive pricing plans which include everything you need to set up and run a fully functioning communications system.

The pricing structure removes any requirement for per minute or per trunk call charges which can quickly add up. Nextiva also offer a free trial, subscription plans to suit the specific requirements of the business and even a free version of their SIP trunking solution.

Support 88%
Features 82%
Cost 91%
  • Cost Savings
  • CRM Integration
  • Eliminate Basic Rate Interfaces

ShoreTel have been established since 1996 and they offer a comprehensive, unified communications system which has been built around their award winning IP phone solution. ShoreTel will guarantee that they can beat any price on their solutions over any five year period and if you find a solution that is cheaper elsewhere they will match it.

ShoreTel is not a VoIP provider as such because they only supply the equipment that you need to purchase a SIP trunking service from another VoIP provider. However, with the equipment already provided, your costs for the SIP trunking will be much lower. The equipment provided by ShoreTel includes phones, an IP PBX and all of the required software to set up and run unified communications systems.

Support 95%
Features 82%
Cost 93%
  • U.S. Based Support
  • Free Trial
  • Scalable

Flowroute offers both SIP trunking and VoIP services to their customers in addition to a wholesale VoIP service to other providers. The company allows businesses to scale their services and tailor them to the requirements of your business. They also offer a range of tools and resources to manage your account yourself.

The main services provided by Flowroute include SIP trunking and Direct Inward Dialing. As the company is a wholesale provider they can deliver comprehensive support to all types of business, from a small local company through a large, international organization. Pricing is reasonable but it is in addition to inbound and outbound calling. Customers can also benefit from toll free numbers, caller ID and porting if you decide to move to another provider.

Support 95%
Features 97%
Cost 93%
  • Cost Savings
  • Local Numbers
  • Excellent

As one of the leading companies in this industry, ClarityTel has helped a number of businesses transform their telephone systems using the most advanced technology. The company takes great pride in their customer service and call quality standards, offering exceptional call quality using their network which spans across the entire country.

They have their own pricing models for SIP trunking and a range of advanced functionality this company is most suited to the small and medium sized business owner. Their pricing plans are reasonable and they offer 24 hour, 7 day a week customer service so you are covered if you need help with your phone system out of hours.

Support 95%
Features 90%
Cost 93%
  • Free Installation
  • Great Prices
  • Increase Network

MegaPath allows businesses to streamline IT management through the combination of voice and internet access while reducing costs. The plans offered by MegaPath allow customers to purchase what they need which is based on the calls that your business will process.

Broadband can be purchased from the company or you can integrate the system with your own broadband provider and the system also comes with a router/IAD to support QoS, security and quality monitoring to ensure the best call standards. What’s more, with MegaPath you can use your existing telephone equipment and IP network which enables businesses to save costs. Calling plans start at 500 minutes per month and you can carry over any allowances you haven’t used to the following month.

How Does SIP Trunking Work?

The best way to explain what SIP trunking is, is to first explain what it isn’t and the problem it is primarily designed to overcome.

If you’re using a hosted VoIP solution, the next logical step to consider is how  your business premises will be connected to the cloud hosting service. In basic terms, businesses have two options when deciding how they connect to their cloud VoIP service provider:

  1. The simplest option is to route everything through the business’s existing Internet Service Provider – the exact same service you already use to send and receive emails and surf the Web: the obvious benefit here is that the service can be set up without disrupting existing internet services or having to switch providers.

However, the potential downside of routing your business VoIP through the existing network is that because the VoIP call data will have to traverse this additional network in the chain before going to the service providers network, there is an increased chance of connection and call quality issues.

If your LAN (Local Area Network, the data network within your building or office) has limited bandwidth it may not be able to support additional heavy internet usage if you’re integrating your VoIP service into your existing internet plan. The solution is to establish a separate network devoted solely to your VoIP phone activity.

This is where SIP, or Session Initiation Protocol trunking comes in:

  1. SIP trunking is an additional feature offered as part of many business VoIP solutions. A SIP trunk provides a new, dedicated ‘direct line’ to your VoIP service provider (and by extension, your hosted PBX) that bypasses your regular ISP. With a SIP trunk, you no longer need to rely on your regular Internet connection for carrying VoIP traffic, ensuring your VoIP network is optimized for call quality and reliability.

Unified Communication and telephone services are delivered through SIP trunking using an IP network as a substitute for the normal telephone system. Using SIP trunking a VoIP user is able to make calls to any phone which belongs to a PTSN network without the need for telephone lines. Connections are established using a suitable hosted PBX solution.

There are three elements required for a SIP trunk to function properly. The first is the SIP provider who supplies the service, the second is a phone system with SIP enabled capabilities and thirdly a border element. This border element enables the connection between the enterprise network and the PTSN or an IP network.

The elements are often managed by the SIP trunking provider and they can be either a switch or firewall. Using these three basic components, a business is able to significantly improve its reach in terms of communications. If an IP-PBX has already been installed this will facilitate the process even more.

Advantages of SIP Trunking

With a SIP solution, businesses can benefit in a number of ways. They are particularly useful for smaller businesses who want to convey to their customers that they are an established enterprise. This can be achieved through the implementation of an advanced communications system such as SIP trunking.

Using this technology also allows small businesses to come across as being more professional. The solution can also work for larger businesses to because SIP calls are locally based so customers don’t have to dial an anonymous call center telephone number.

Other benefits include:

Cost Savings

When you add a SIP trunk, you eliminate the need for separate PRI connections to your networks. (PRI, or Primary Rate Interface, refers to a physical phone line that only facilitates voice transmissions; it can’t support data).

However, with SIP, you can consolidate your separate voice and data circuits, and purchase only the trunks that you need.

Optimal use of bandwidth

SIP trunks smartly allocate bandwidth to ensure that both voice and data can be supported.

Additionally, some providers use voice compression to make efficient use of connections without compromising call quality.

Increased flexibility

When a SIP phone system has been set up, businesses can add or remove as many telephone lines as they require without having to pay for more lines than they need or waiting for the telephone company to add more lines when the business expands.

All features of the system can be easily controlled through a dedicated web portal and the systems can be tailored to meet the specific communication requirements of each business

Consolidate communications providers

Many IP PBX providers also offer SIP trunks. When the services are combined, the customer can take advantage of a more robust range of voice, data, security and other services.

Plus, dealing with only one communications provider is easier than juggling the service requirements of many.


Many trunks located on an analogue system can be affected by the weather and depend on costly telephone companies to sort anything out when it goes wrong. SIP trunking can avoid any scenario such as this. When a fault occurs the system can reroute the communication to an alternative line and priorities certain phones or networks


Traditional phone systems require lots of hardware which is costly to install and maintain. A SIP trunking system can adapt to your business without having to add any new hardware and adding new lines doesn’t depend on the installation and setup of new devices


If you move to a bigger premises or relocate your office for some other reason you can take SIP trunking telephone numbers with you. As the phone lines operate from the internet you can take the telephone numbers to wherever you go.

International Calling

If you conduct a lot of business overseas, SIP systems are an affordable solution with some providers offering free international calls

Highly Scalable

SIP trunks allow businesses to easily and affordably add circuits as they grow. You simply add what you need, when you need it.

Choosing the right SIP Provider

Once you have researched SIP Trunking sufficiently and you have decided it is the most suitable option for your company, the next important step is to determine the best provider for this service. Look at all providers carefully to see how each of them meets the requirements of your business and whether their solutions fit in with what you need to run the business effectively.

There are many things to take into consideration before you settle on a SIP provider including;

    • Server – It is important to determine whether the provider uses their own networks to deliver the SIP service or whether they use a third party
    • Security – Always a concern for every business owner, calls that you make and receive over the internet must be secure. Check with the provider what security features they have in place to keep your calls secure. As a minimum you should ensure that the provider offers a secure SIP trunking solution featuring anti-fraud systems and protected networks in place as well as active risk monitoring so any security issues can be quickly identified and resolved
    •  Porting – If you decide to leave the company for whatever reason check that it offers the facility to port your number to a new provider
    • Customer Service – There is nothing worse than using a provider who has poor customer service or technical support that is seriously lacking. Although SIP trunking is great at rerouting calls when something does go wrong, make sure that you can contact your provider quickly. Find out how you can contact them, their opening hours and whether they have any out of hours customer service support if you need it.

Improving your business communications

Conventional phone systems will make use of a physical telephone line and circuits to deliver services which are connected to the PTSN, whereas a SIP trunk will replace these physical connections with a virtual connection using an IP network. Due to the way in which information is transferred the SIP system can easily handle a variety of features such as internet services, video content and audio. Put simply, SIP Trunking is used to create a voice connection on a data network.

The introduction of PBX and VoIP systems, voice communications could function effectively without traditional landline telephones and circuits. Using the SIP system, signaling languages were designed to carry data over the internet which significantly reduces the cost and eliminates the restrictions of a traditional phone system. This allows businesses with an IP PBX system to connect with other networks outside the business through the PSTN (Public Switched Telephone Network).

SIP Calls don’t require phone lines and they are offered by leading SIP providers for businesses who require small and large scale phone systems. They are also cost effective and create a more professional impression for small businesses.

Getting More Flexibility

As a great alternative to the old and restrictive telephone connections, SIP trunking offers businesses much more freedom in terms of their communications. With the old systems if you required an additional trunk to accommodate a growing team or to set up a dedicated customer service team, businesses would have to purchase a further 24 lines as a minimum, even if they only required one or perhaps two additional lines. Consequently, businesses wasted an enormous amount of resources, paying for 23 or more lines that they simply didn’t need. One of the best advantages of the SIP solution is that it is offers great scalability allowing businesses to both add or remove lines as their business changes.

When it comes to SIP trunking services, the system allows video, data and voice calls to be easily transferred using the same telephone line so local, long distance, broadband and video calls are all managed using the same network. Provided that you have a reliable internet connection and suitable bandwidth all of the required resources will be in place for basic web use while maintaining the required call quality.

Reducing Costs

As all of the services you need will function on the same network, the hardware that you require will reduce considerably. Not only this, your costs will also reduce because you won’t have to manage a dedicated IT manager to monitor your phone lines and using SIP technology you don’t have any hardware on site which means that you can rent a smaller office space, generating further resource savings. The SIP provider will set up a proxy server to manage incoming, outgoing and PBX calls and they can even manage all technical aspects of running the system too.

Relying on a traditional telephone system would often result in service outages which have the potential to cause major disruption to business owners. With SIP trunking businesses no longer have to worry about this because the SIP trunks will automatically redirect the call to another extension or location, eliminating downtime. If there is a power cut calls can even be routed to a separate satellite office in some instances.

Final Word: Why Choose SIP Trunking?

There are certainly many other alternatives to SIP trunking so why should you use it instead of one of the other solutions? Firstly, SIP phone systems allow businesses to save money which can be used in other areas of the business.

SIP trunks are almost a quarter cheaper than the standard PRI trunks and that saving is made even before you start factoring in the cost of calls. As well as the flexibility that you just can’t find with traditional phone lines, SIP allows businesses to make free voice calls between any office using the same VoIP system.

Other alternatives to SIP are the Primary Rate Interface Trunking (PRI) which provides the resources for large businesses to meet demanding communication requirements, operating from multiple locations. However this system is often deemed to be an old and out of date option in comparison with SIP.

In addition, the PRI trunking depends on T1 lines where circuits have to be added and calling fees cost a lot more. The PRI systems can also fail during poor weather conditions, which is not ideal if the business is relying on it as their main method of communication.

SIP Trunking is an affordable, reliable and resource saving solution which can help businesses enhance their communications, increase call quality and deliver an improved standard of customer service.

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Jack is a senior writer at with over a decade's experience researching and writing about consumer technology, from security and privacy to product reviews and tech news.