SIP and SIP Trunks

Confused by SIP trunking and wondering how it works? SIP stands for Session Initiation Period and is the fastest-growing telephony service. It’s reliable, cost-effective and many businesses prefer it to traditional ISDN because it is cheaper and more flexible.  SIP trunking means that business phone systems can operate using an internet connection, rather than a traditional business phone line.

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Session Initiation Protocol (SIP)

Here at 4Com, we simply connect your business phone onto our network through a broadband or Ethernet connection. All your inbound and outbound calls are then carried via the network connection.

SIP is one of the key parts of a VoIP phone call. As the name suggests, SIP helps to start the call and can also be used to start video or other media transfer over the internet. SIP will look up the number you have dialed, find the device you are seeking to connect with and establish the connection once the recipient has answered their phone.

SIP Trunking Features

There are a number of great and practical features which make SIP trunking so popular with businesses. A few key features are:

  • SIP trunks are virtual so don’t require any additional hardware, making it more cost-effective
  • This also means employees can work from anywhere in the world and still have a local telephone number
  • And if you want to keep a local number when moving, SIP won’t cost you anything extra in call forwarding
  • In case of emergency and employees not being able to make it into the office, you can set up SIP trunks to be rerouted to other locations
  • SIP offers increased phone service reliability
  • It can deliver multiple voice lines to a single organisation
  • As your business grows, it’s easy for you to simply add more SIP trunks via a SIP provider instead of installing a new telephone line

What is a PRI trunk?

PRI stands for Primary Rate Interface and is used as voice recognition technology that delivers data into a businesses’ VoIP PBX via a circuit line.

Simply put, PRI is a service that transmits voice from the phone in your central office location to the phone in your location.

It is considered old-school telephony, due to the fact it is physical hardware and not transmitted via the internet like SIP trunking is.

What is the difference between SIP and IP?

Before we explain the differences, it’s important to know what IP is. IP stands for internet protocol, and IP phones use the internet to digitally transfer voice from one phone to another.

A key difference between IP and SIP phones is their phone handling ability. IP phones can do the necessary – meaning they can handle a voice call from person to person – but SIP phones have the capacity to do so much more.

Most service providers, including 4Com, offer a combined technology of IP and SIP so that your business phone line has everything it needs.


With ISDN (Integrated Services Digital Network), businesses are required to pay separately for voice and data services, and it uses the traditional copper network to deliver voice and data. Comparatively, SIP is a more modern telephony solution that delivers voice and data via the internet.

So, what do you choose for your business? SIP trunking or ISDN? Take a look at some key points below and discover which is best for you:



  • ISDN has been proven to have a reliable and high-quality voice call service
  • It is flexible and can be added to new or old telephony systems
  • It provides a high data rate


  • It is, on average, more costly than other business telephone systems
  • If your business system is offline or unavailable, there are limited options with ISDN to keep your inbound and outbound calls running



  • SIP trunking is highly customisable for businesses, and you can either scale up during busy call periods or down in low call volume periods
  • SIP is easy to manage and maintain
  • Due to its virtual nature, SIP is future-proof and allows you to easily hold virtual meetings and send video messages


  • As it relies on internet, you will need to ensure you have a fast and reliable connection

However, when making your decision please bear in mind that ISDN is set to be discontinued by 2025. If you are looking to upgrade your system, where possible we would recommend for you to change to a SIP system. It will prevent you from the effort of having to change further down the line.