What is SIP trunking?

If you're looking for ways to cut costs and streamline your business phone systems, SIP trunking may be the answer. Learn more about the process here

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We are a team of writers, experimenters and researchers providing you with the best advice with zero bias or partiality. This article was authored by:
  • Scarlett Cook

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

But what exactly is SIP trunking? And how could your business use it on a day-to-day basis?

Alternatively, you may have already heard of SIP trunking, but need a more detailed understanding in a clear and concise format. We’re here to provide the key information you need to know, to help you understand this significant aspect of phone systems.

This article offers a guide to the main areas you should know about to help you assess if SIP trunking is right for your business, including what it is, how it works, and why your business should consider using it.

Plus, we’ll offer advice to help you get your network SIP trunk ready, including how to choose a provider and a rundown of the best providers out there. We’ll also give you an idea of one of the most important factors: cost.

In this article, we’ll cover:

What are SIP trunks?

Session Initiation Protocol (SIP) is a way of making phone calls via the internet. In order to use SIP trunking, you'll need:

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

As part of this, you’ll need a firewall to ensure that the SIP trunking is secure – ideally one dedicated to this function. This firewall can be offered by your provider, or organised by your IT specialist or team (if you have one).

As well as phone lines, SIP can also enable other forms of communication, such as email, chat, and text messages. You can check with your SIP provider to see what they offer.
This is because SIP allows UC – a way of connecting all communications into a single channel. It can offer an ID or number that isn’t specific to one particular channel or device, giving you and your team greater freedom.

When you think of making phone calls, it’s likely you think of outbound calls, whether that’s to clients, customers or suppliers. However, SIP trunking can also be used for internal calls too. Plus, you can allocate the exact number of phone lines you need – there’s no set amount, as was the case with previous analogue lines (e.g. 15 or 30 lines).

Although it’s possible to run all of your business’ internet services through the same internet connection, it could be preferable to have an internet line solely for the phone and SIP communication, depending on your company’s usage and data requirements.

Similarly, there’s the option of using a physical PBX (or your old/current phone system) in conjunction with a SIP trunk, using a gateway that allows them to interact. However, it may be better to have all internet-based IP PBX, with SIP going through a separate internet connection to help ensure better call quality.

Essentially, SIP trunking has two main functions:

  • To connect your business’ phone systems to the PSTN (Public Switched Telephone Network) via an internet connection
  • To allow VoIP telephone systems to work

An IP PBX 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.

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.

Why use SIP trunking?

In this section, we outline some of the main reasons why businesses might 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 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 back-up. You can reroute your calls to another SIP trunk with the same provider, meaning there’s no need for a separate back-up 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.

What are the key benefits of SIP trunking?

  • Cost-saving potential (read our page on SIP 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

Is your network ready for SIP trunking?

While you may have heard about SIP trunking and are interested in using it in your small business, how do you know that your network is capable of managing SIP trunk connections? Here, we offer a guide 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

Top things to know 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 profile 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 – this is 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?

Who are the best SIP trunking providers?

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…

ProviderBest forPrice range
BTBest for network£8.95+ per month
GammaBest for infrastructureContact provider
SpitfireBest for affordability£4+ per month

Check out our guide to the best SIP trunking providers for a more in-depth look at the top providers.

Or, for more information based around your business’ specific needs, simply complete the form at the top of the page to compare quotes for telephone systems.

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 which 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

Read our SIP trunking pricing page for more detailed information.

Next steps

From reading this article, you’ve learned more about what SIP trunking is, and how it works in a small business setting. We’ve included our top tips for choosing a SIP trunk provider, and identified our top three picks.

Next, read our guide to the best SIP trunking providers for more detailed information.

Alternatively, fill in the form at the top of the page to compare quotes for telephone systems now.

Scarlett Cook
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