What is SIP trunking?

If you’re a small business owner looking to minimise the costs and infrastructure necessary to run your company’s phone system, SIP trunking could be the answer.

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Session Initiation Protocol (SIP) trunking is a way of making phone calls via the internet. It is installed as part of an internet connection and is used to connect to the Public Switched Telephone Network (PSTN). In order to use SIP trunking you’ll need:

  • A secure, reliable internet connection
  • A VoIP or PBX phone that’s SIP compatible

SIP trunking is becoming ever more popular, as the need for analogue lines becomes redundant for many businesses who have ditched their traditional office setting for a flexible, remote working environment.

But that’s not to say this system will suit every business, which is why we’d recommend you use our custom-built cost comparison tool to help you move forward. Developed by our team of SIP trunking experts, this tool will help you make an informed decision about which product best fits the needs of your business, offering you personalised bespoke quotes from the UK’s top telephone system providers.

Alternatively, read on for our extensive guide to SIP trunking that highlights the information you need to know before assessing if SIP trunking is right for your business – including what it is, how it works, and why your business should use it.

How does SIP trunking work?

Session Initiation Protocol (SIP) is a way of connecting phone calls virtually, as opposed to using real-life phone lines.

A SIP trunk is installed as part of an internet connection and is used to connect to the Public Switched Telephone Network (PSTN). This allows phone calls to be sent and received digitally.

The combination of Voice over Internet Protocol (VoIP) and SIP creates an internet-based phone connection. This is in contrast to traditional phone systems, which use physical phone lines.

Essentially, SIP trunking has two main functions:

A PBX SIP trunk phone system can be connected in one of two ways:


With this option, it’s possible to keep the system at your business’ site, in the same place your server is stored. This is more likely to suit bigger companies with the infrastructure and resources necessary to maintain it.


Alternatively, it’s possible to use an external provider to host the system on the cloud for you. This may be a better setup for startups, as it is generally cheaper and easier to organise.

Note that with an on-premises PBX, it’s possible to use SIP trunking on an existing phone system. This can allow for multiple calls at the same time, with the possibility of unlimited calls.

How is call quality ensured?

It’s possible to use a leased line, which is only for voice data. This means that voice data (i.e. phone calls) are given priority.

There is also the Quality of Service (QoS), which prioritises voice traffic via network routers. It refers to the features and techniques that prioritise and allocate bandwidth — your SIP provider should take care of this.

Why use SIP trunking?

In this section, we outline some of the main reasons why your business would want to use SIP trunking.

Manage multiple phone lines

While SIP trunking and call centres seem to go hand in hand, other types of businesses could benefit from using SIP too.

For example, if you run a recruitment agency and have multiple team members speaking with clients and staff, SIP trunking could help you handle high call volumes more efficiently. In fact, SIP could be ideal for just about any business that requires multiple phone lines!

As SIP trunking can be used internally or externally between two or more people, it’s possible to use it for conference calls, too.

Grow and scale easily

But what if you’ve just launched your business, or you run a startup with only a few workers? It’s still useful to consider how SIP trunking could work for your business in the future.

Since these phone lines operate over a virtual connection, there’s no need to worry about arranging a physical location for them on your premises, or setting up new phone lines if you move office. This means your phone network can easily grow and scale with your business – and you can keep your old phone numbers, too.

Alternatively, you may run a seasonal startup or a company with a varying workload, such as a customer service business. SIP trunking allows you to scale your phone lines accordingly.

Plan for worst-case scenarios

While in the early days you’re particularly likely to be focused on growth and development, it’s still essential to consider various worst-case scenarios.

For example, what does your disaster recovery plan look like? If your phone lines were to go down, SIP trunking can offer an instant backup. You can reroute your calls to another SIP trunk with the same provider, meaning there’s no need for a separate backup system.

Streamline communications

While we’ve focused mostly on phone calls, SIP trunking can be used for other types of communication as well, such as email and messages. Some SIP providers may even offer these services themselves, so you can unify your business’ communications.

Similarly, as your business develops, you could use SIP trunking to connect staff in multiple locations. This offers a cost-effective method of team communication, requiring only an internet connection and appropriate phone devices.

SIP trunking can also be used to make your calls go out from a specified location, as well as make calls from mobiles go out as a chosen number. SIP uses a single connection to manage multiple phone numbers, without multiple lines.

The key benefits of SIP trunking, summarised:

  • Cost-saving potential (read our page on SIP trunk pricing for a more detailed look at the costs involved)
  • Less space required
  • Scalability
  • Flexibility

Who is SIP trunking suitable for?

SIP trunking can be deployed in a number of business settings. However, the following examples highlight the types of business requirements that SIP trunking might be most suitable for.

  • Overseas calls – internet-based SIP trunk connections provide a cost-effective solution for businesses that make a high number of international calls. Some providers even allow users to display local numbers for the specific region they’re calling
  • Seasonable demand – as SIP trunk lines can be easily added or removed, businesses can develop a tailor-made phone system that can be adjusted depending on demand
  • Unified communications – a SIP trunk connection can allow for various forms of communication, including voice calls, video calls, and messaging functions. This is ideal for businesses that may need more ways to communicate besides standard telephoning

Ensuring your network is ready for SIP trunking

While you may be interested in using SIP trunking within your small business, how do you know that your network is capable of managing SIP trunk connections? Here, we offer some top tips to help you prepare:

  • Internet connection – your business will need a strong, reliable internet connection to run SIP trunking. It could also be useful to have a dedicated connection solely for SIP trunk usage
  • Bandwidth amount – check how much bandwidth will be required to use your chosen SIP trunk provider service
  • Phone system – your phone system will need to be able to support the network and SIP trunk service
  • Review contracts – if you’re already using SIP or another service from a phone system provider, it’s worth checking for any conditions that may exist in your current contract
  • Security protection – while it’s likely that your startup already has firewalls and anti-virus software, check if these will be sufficient for SIP trunking as well. Also, SIP trunk providers often offer these features themselves, which can be a good way of determining which provider is best for your business

Things to consider before selecting your SIP trunking provider

With so many providers out there, how do you find the best match for your startup’s needs? Here we highlight some of the main factors to consider before making that all-important decision.

  • Security – what security is required, and does the provider offer this?
  • Testing – are network or bandwidth tests necessary, and will the provider run these?
  • Compatibility – is compatibility equipment (such as a gateway) required, and if so, is it available from the provider?
  • Uptime – a way of measuring how reliable the service is, with 99.999% uptime serving as a high standard
  • Plans – what’s included in the monthly package (e.g. number of minutes, specific international regions), and are there any additional charges?

The best SIP trunking providers: an overview

Whether you need a SIP trunk that operates from a solid network, has high-quality infrastructure, or is an inexpensive choice, our list of the best providers has something for all businesses. Discover how they compare in the table below…

The best SIP Trunking Providers of 2021

ProviderPricing fromBest forSetup
BT£13 per monthRemote working businesses needing a reliable, secure, cloud based SIP phone systemFree
GammaContact provider for quoteBusinesses needing flexibility to upsize or downsize lines whilst retaining the same contact numbersFree
Spitfire£4 per monthSmall businesses wanting an end-to-end business phone solution - including phone system, data circuit and SIP all managed together Free
TalkTalk BusinessCall bundle or Pay-As-You-Go plans – contact provider for quoteBusinesses requiring a high level of customer support, with 24/7 support available from UK-based agents. Free
Xinix£4 per month Affordability, with cheap monthly costs for its plans, and reasonably priced product add-ons for international numbers (£5 per month). Also 30 day free trial. Free
Zen£30 per month Integration with a number of telephone based systems, including Mitel, Avaya and 3CX£25 connection fee

All pricing is correct as of 16th August 2021 

You can check out our review of the best SIP trunking providers for an in-depth deep dive into these top providers.

What if I'm still stuck choosing between two providers?

We’ve done our best to summarise the best SIP trunking providers for small businesses, listing their features and highlighting their pros and cons.

And although we can’t choose your provider for you, our team of experts have put together an incredible comparison tool to help you choose the right service for your business, even if you are torn between two of them.

Just fill out a few details, and we will get back to you with quotes and additional information.

How much does SIP trunking cost?

There are a number of elements to consider when working out how much it will cost your small business to install a SIP trunk system. These include:

  • Gateway – if you’re planning to use an existing, traditional phone system, or an IP system that isn’t from the same provider, you may need to use a gateway to make them compatible with the SIP trunk
  • Equipment – you’ll need to ensure that your business has phone systems that are compatible with SIP, so if you want to use these and not the gateway option, then be sure to factor in the cost of installing new phone systems
  • Monthly plan – the SIP trunk service is usually paid for on a month-by-month basis, with costs varying depending on the type and level of service you opt for
  • Additional charges – these are likely to arise from your SIP trunk provider’s service, such as costs for specific phone numbers, as well as costs per channel and per minute to make calls

What next?

From reading this article you’ve learned that SIP trunking is a way of making phone calls via the internet, and is great for small businesses because of its scalability, flexibility, and low cost. We’ve included our top tips for choosing a SIP trunk provider and identified the best providers available in 2021, including BT and Gamma.

Regardless of our opinion, you should always compare SIP trunk providers before signing on the dotted line. The last thing you want is to be stuck in a contract with a service that doesn’t fulfil the needs of your business, even if it is, generally speaking, a great product.

This is where our custom-built comparison tool comes in. We’ve partnered with some of the best SIP trunk phone system suppliers to help you get the best service for your business. Simply provide us with a few basic details, and providers will be in touch with bespoke personalised quotes for you to compare, along with further details about why the service they offer would best suit the needs of your business.

Written by:
Ross has been writing for Startups since 2021, specialising in telephone systems, digital marketing, payroll, and sustainable business. He also runs the successful entrepreneur section of the website. Having graduated with a Masters in Journalism, Ross went on to write for Condé Nast Traveller and the NME, before moving in to the world of business journalism. Ross has been involved in startups from a young age, and has a keen eye for exciting, innovative new businesses. Follow him on his Twitter - @startupsross for helpful business tips.
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