SIP Trunks; what are they, and how do they work?

SIP Trunks are a vital part of VoIP and telecoms today, so in this guide, we look to tell you everything you need to know about the technology, what it is, how it works and when you do and don't need to use it.

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BT is switching off its PSTN and ISDN networks in 2025, so all businesses (and consumers) will need to switch to VoIP at some point before then. With SIP being integral to modern telephony, anyone moving to an internet-based phone system will come to rely upon it.

What is SIP Trunking?

SIP is a protocol, which in layman's terms, is a set of rules and procedures used to transmit data between computers. Much like HTTP is the protocol used to deliver web pages on the internet, SIP connects and maintains VoIP calls.

An acronym for Session Initiation Protocol, SIP is a signalling protocol used by IP telephony to establish, maintain and terminate calls. While it plays a significant role in VoIP telephony, it isn't the protocol used for the actual call transmission, that is done using RTP (real-time transport protocol).

So what does SIP trunking mean?

The "trunking" part of the term is a bit of a misnomer, as "trunk" used to be used for a collection of phone lines shared between users. SIP trunking refers to a virtual link between your IP enabled telephone system and the PSTN via an internet connection. Another way to think of it is a SIP channel or line; the more you have, the more calls you can make at the same time.

While in the past, businesses used ISDN to connect to the wider PSTN, today, with IP telephony, SIP Trunks are used for that purpose, with the connection being made over the internet. 

Much like businesses in the past would have multiple ISDN channels to allow more than one simultaneous call, today, you have one SIP trunk per active call.  

Tip: Most businesses will need approximately one SIP trunk per three people.

How does SIP work?

For a deep dive into the history of SIP and how it works, this guide is an excellent resource. For those of your short time or looking for a less technical roundup, here is a summary of how the technology works.

  1. Using an IP enabled handset or softphone, you start a phone call.
  2. Your IP PBX uses SIP to set up (connect) the call.
  3. Once the call is established (called a handshake), data is sent and received using RTP.
  4. SIP is then used to manage the call until it is terminated.

All of this is happening in milliseconds, and the result is you get a higher quality and more reliable call than that of PSTN or ISDN.

How SIP Calls are made

Hosting the call

While crucial to VoIP telephony, SIP is merely one part of it. You'll require hardware and software (endpoints) at either side of the call, along with an IP PBX to make VoIP calls. 

Our hosted PBX removes the need to worry about SIP trunks altogether as you pay on a per-user basis for the service, and we take care of the rest.

If you plan to have an onsite phone system and want to learn more, please read on.

How many SIP trunks do I need?

Each business is different, but for most, we recommend one SIP trunk for every three people using the phone system. Of course, if you operate a call centre or have a team that is on the phone a lot, you may wish to have more than this. The great thing with VoIP and SIP is that as long as you have enough internet bandwidth, you can scale the number of SIP trunks you have as high as you like. 

Based on the above, we'd therefore recommend something like the following:

  • Number of SIP Trunks for 3 people: 1
  • Number of SIP Trunks for 9 people: 3
  • Number of SIP Trunks for 15 people: 5
  • Number of SIP Trunks for 30 people: 10
  • Number of SIP Trunks for 60 people: 20

The critical thing to remember is that as the calls are made over the internet, each of them will use bandwidth; the more calls you have, the more bandwidth you require.

The VoIP phone system from T2k has saved us serious expenditure and has changed the way we work for the better.
Steve Knell -Altogether Care
on Google

How much bandwidth do I need for SIP calls?

We recommend that you have at least 100Kb of upload and download bandwidth to make a high-quality VoIP call. That means you need at least 1Mb in both directions (up and down) for ten concurrent users (SIP trunks). Because of the relatively predictable bandwidth profile, it's pretty easy to work out how much capacity you need if you only use the internet connection in question for SIP and VoIP. The trouble comes when you also use the line for general internet browsing and web activity, as it's all too easy for someone to start downloading a big file and all of the bandwidth to be gobbled up for a few minutes. This sudden maxing out of the connection by someone's computer will have a significant detrimental effect on your calls, which is why we usually recommend that you have a separate connection for your phones and computers, regardless of the size of your team. 

Who should consider using SIP Trunks?

Any business that has or is planning to have an onsite IP PBX will need SIP Trunks; they're essential to the operation of your phone system. Of course, if you opt for a hosted phone system, you may not need them at all. 

Here are the types of businesses that typically opt for an on-premise phone system over a hosted solution and therefore will need SIP trunks:

Call centres

SIP Trunks used in a call centre

Call centres with high call volumes, be that sales, support or customer services teams, often benefit from an economy of scale when it comes to phone systems. Because they have so many users, they can justify the initial outlay and ongoing maintenance of physical equipment. Of course, this isn't always the case, and we know of plenty of call centres that opt for a hosted solution, not least to help with cash flow.

Multi-site companies

Companies that already have multiple offices and have invested in leased lines, MPLS and other types of business-grade connectivity will often opt for an onsite system with SIP trunks. Over the life of the phone system, they will usually get an ROI in comparison to using a cloud-hosted service. As these companies already have the infrastructure in place, it's often a no-brainer to buy more equipment and pay for some SIP Trunks rather than going down the hosted route. 

Companies with in-house technical teams

Some small businesses still opt for an onsite system with SIP trunks even with very few users because they have the in-house technical capabilities to set it up and maintain it themselves. Of course, this is relatively rare, and usually, it's the IT support companies, web developers and co that go this route, but it still is possible and does happen.

Why are businesses using them?

Putting the fact that ISDN and PSTN services are to be discontinued before long to one side, there is a range of significant benefits to moving to SIP and indeed VoIP.

We should point out that many of the benefits listed in this section aren't reserved for onsite IP PBXs and SIP; most if not all apply to hosted telephony.

Reduced costs

If you've been in business for some time, you'll know that ISDN lines are typically expensive, both to install and from an ongoing rental perspective. Not only that, you often end up paying for capacity you never use with ISDN. SIP Trunks tend to be far more accurately packaged around your business's requirements and actual usage. 

Once you're set up with SIP Trunks via your VoIP service provider, you'll enjoy a significant reduction in call costs and line rentals, which, whether you're a large or small business, will be welcome news; we're sure. 

Increased functionality

After cost savings, probably the most significant advantage of SIP and VoIP as a whole is the extraordinary range of functionality modern phone systems, be that hosted or onsite provide. Here is just a summary of what you can expect from even relatively low-end systems:

  • Music on hold
  • Auto-attendants
  • Built-in call recording
  • Hunt-groups
  • Voicemail to email
  • Call routing
  • Instant messaging
  • Conference calling
  • And much more

For many companies, the way to get the most functionality without having to invest a significant amount into an onsite solution is to get a hosted business phone system. Platforms such as our Horizon service benefit from being continually updated, so you get the latest features and functionality without having to pay anything more a month.

Remove geographic restrictions

SIP trunks and VoIP, for that matter, free you from being tied to telephone numbers that are local to you. You can get a VoIP phone number in any town or city in the UK; you can even get international ones! This is great if you have a national or global customer base, and better yet, if you ever need to move your operations, you can keep the exact numbers, regardless of where you end up.

Unified Communications

To many, the term Unified Communications sounds quite abstract,  but it simply means a telephony solution that "unifies" various communication methods. Being that the phone system is typically central to Unified Communications, SIP Trunks are too. 

Whether you opt for an onsite IP phone system or a hosted solution like Gamma Horizon, you'll be opening the door to Unified Communications and taking strides towards a more productive workforce. 

The PSTN and ISDN switch off

With the UK's PSTN and ISDN networks being switched off by BT in 2025,  anyone that would like to retain a form of landline will need to move to VoIP. While many will opt to move to a hosted PBX, many will choose to invest in an onsite SIP-based telephone system. For those that do, SIP Trunks will play a key role.

How to switch to SIP

The process of arranging for the installation of SIP Trunks and enabling VoIP services should always start with research and speaking to a leading VoIP provider. When you talk to a company like ourselves, our telecoms experts can provide you with personalised advice to ensure that your business gets the best value for money but the best service for your requirements. 

Modern office using telecoms and SIP

Things to consider when moving to SIP

Whether it's before or after your initial conversation with us, you'll need to give some considerations to your company's requirements so that we can help you choose the best way forward. Here are some of the key things we'll need to know from you:

How many concurrent calls do you expect to have?

Probably one of the first things to consider when deciding on how many SIP Trunks you need is getting an understanding of how many active calls you and your team are handling at any given time. An active call can be both inbound or outbound. While for many, a ratio of 1 SIP trunk to 3 people is sufficient, if you operate a high volume, customer-focused company with an active sales team or a call centre environment, you'd likely need more. 

How many minutes do you need?

Many providers, ourselves included, will bundle minutes with your package, but getting a rough idea of how many you need is essential. Check your historic bills or get in touch and speak to one of our specialists who will be able to give you approximations. 

Would you like to move (port) any of your existing numbers?

Many companies see their telephone numbers as an extension of their identity and brand, so keeping them and moving them when you switch to SIP is essential. The process of moving telephone numbers is called "porting" and consists of you providing your existing phone provider with a Letter of Authority, instructing them to move the numbers to your new SIP service. 

If you plan to keep your numbers, please let us know when you speak to us, and in the meantime, here's an editable Customer Letter of Authority template that you can use. 

Joining T2K is one of the best if not the best business decisions we've made.
Paul Crisell - Vale Veterinary Clinic
on Google

Do you need any new phone numbers?

VoIP phone numbers are often cheap, sometimes even free, so if you need any other numbers, let us know, and we'll let you know of the cost, if any. Since SIP and VoIP free you from the geographic restraints of telephone lines, you can get a telephone number anywhere in the world, which is helpful for many reasons. 

Do you plan to change the way you and your team works shortly?

When choosing between SIP Trunks and a hosted VoIP service, it's essential to spend a bit of time considering whether how or where you work might change shortly. Suppose you are thinking about allowing people to work from home more frequently or have employees who regularly move between physical locations. In that case, SIP Trunks and an onsite phone system might not be the best fit for your business. It may be better to look at a cloud-based phone system so you're not tied to a piece of hardware.

What connectivity do you currently have?

Whether you have a hosted PBX or an onsite IP phone system, your connectivity, be that between sites or to the internet, will be an important area of consideration. In almost all cases, we recommend that you have a separate internet connection for your telephone calls to ensure that quality isn't affected by general internet browsing and streaming. There are, of course, exceptions to this, say you only have a couple of users, or perhaps you have a dedicated leased line with significant capacity for this not to be a concern. Again, when you speak to us, we'll ask about your internet connections and develop a plan specific for you. 

Do you have space on site for a phone system?

Most businesses do, but of course, if you only have a small office, you may not have the physical space for a telephone system and the associated wiring. As much as SIP reduces the need for ISDN and PSTN lines, you still need internal networking and connectivity. If you're limited on space or are planning on downsizing your office, again, it might be better to look at a cloud-hosted VoIP solution instead of SIP trunks and an on-premise system.

Choosing a SIP Trunk Provider

With so many SIP Trunk providers to choose from, it can be a little overwhelming to understand the pros and cons of each, especially as, by in large, the service they provide is very similar.

T2k are a telecoms company with over 30 years of experience in working with businesses across the UK. Over the years, we have retained our core key people, meaning that our customers get continuity and the reliability that comes with knowing who they are working with. We aren't a huge multinational with a call centre packed full of people that don't know who you are and anything about your business. When you work with T2k, you benefit from a personal service of a small business but with the reliability and continuity that only comes from decades of being in business. Of course, we're biased, but feel free to look at some of our recent reviews to see what our customers think. 

Lee Clarke
Sales Director

Having worked for T2k for nearly 25 years, it's fair to say that Lee is an expert when it comes to all things telephony and business communications. Overseeing the commercial side of the business, he has helped the company evolve and grow through the decades. In recent years, and with the advent of VoIP and hosted telephony, Lee has made sure that T2k is at the forefront of technological developments. With a firm interest in helping businesses navigate the world of telecoms, Lee is responsible for the majority of the content on this website.

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