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What is SIP trunking?

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This article was originally published on StartUps

If you?re looking for ways to cut costs and streamline your business phone systems, learn more about SIP trunking here

If you?re a small business owner looking to minimise the costs and infrastructure necessary to run your company?s phone system, it?s worthwhile considering SIP trunking.

But what exactly is SIP trunking, and how could your business use it on a day-to-day basis? We?ll provide the key information you need to know to understand this significant aspect of phone systems.

In this article, we?ll cover:

  1. What does SIP stand for?
  2. What is SIP trunking?
  3. Why use SIP trunking?

You can read the whole article to gain a detailed understanding of SIP trunking. For more information on a particular section, click the relevant link above.

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

What does SIP stand for?

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 on an internet connection, and is used to connect to the Public Switched Telephone Network (PSTN), allowing phone calls to be sent and received digitally.

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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.


Now that we?ve covered what SIP is, here?s a quick guide to some of the other related common words and phrases that you may come across when researching SIP trunking.

  • DID ? Direct Inward Dialing number; this is your business? phone number
  • IP ? Internet protocol; a type of data connection
  • ISDN ? Integrated Service Digital Network; an internet connection that uses a phone line
  • ISP ? Internet service provider
  • PBX ? Private Branch Exchange; your phone system (how calls are directed)
  • PRI ? Primary Rate Interface; another way of referring to phone lines that work with a physical line, as opposed to a virtual connection
  • PSTN ? Public Switched Telephone Network; the global group of public telephone networks
  • T1 ? A type of copper or fibre optic line that can be used for phone calls or data
  • UC ? Unified Communications; when multiple communication methods (such as email, phone and text) are connected into one channel
  • VoIP ? Voice over Internet Protocol; sending voices as data across an internet connection

What is SIP trunking?

As mentioned above, SIP is a way of making phone calls via the internet. So what do you need to use SIP trunking?

  • 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 purpose. This can be offered by your provider or organised by your IT specialist or team, if you have one.

While we?re focusing on SIP phone lines, other forms of communication are possible too, such as email, as well as 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.

When you think of making phone calls, it?s likely you think of outbound calls, whether that?s to clients, customers or suppliers ? but SIP trunking can also be used for internal calls too. Plus, you can allocate the exact number of phone lines you need ? there?s not a set amount like previous analogue lines e.g. 15 or 30 lines.

Although it?s possible to run all 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. Read our guide to the best broadband for business for more information.

Similarly, there?s the option of using a physical PBX (or your old/current phone system) and a SIP trunk with a gateway that allows them to interact. However, it may be better to have all internet-based IP PBX and 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 with an internet connection
  • To allow VoIP telephone systems to work

And it takes either of these two forms to connect to an IP PBX:


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 be suitable for 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, which is more likely for start-ups as this is generally a cheaper and easier to organise option.

Note that with an on-premises PBX it?s possible to use SIP trunking on an existing phone system, allowing 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 the voice data (the phone calls) are prioritised.

There is also the Quality of Service (QoS), which prioritises voice traffic via network routers ? your SIP provider should take care of this.

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 start-up with only a few workers? It?s still useful to consider how SIP trunking could work for your business in the future.

As the phone lines operate over a virtual connection, there?s no need to worry about a physical location for them on your premises, or setting up new phone lines if you move office, meaning it can easily grow and scale with your business. Plus, you can keep your old phone numbers too.

Alternatively, you may run a seasonal start-up 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 too, such as email and messages. Plus, some SIP providers may offer these services, 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 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

When might you need a physical phone line?

What are the next steps?

At this stage, you?ve learned more about SIP trunking, including what it is, as well as what it stands for (Session Initiation Protocol) and the ways in which it could be used in small businesses.

Now that you understand what SIP trunking is, the next step is to read our page on best SIP providers for more detailed information on the kinds of services available.


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