SIP trunking - How it works and the benefits for VoIP

SIP trunking works in pretty much the same way as VoIP does – you make calls over an internet connection instead of a traditional landline. And, just like VoIP – or Voice over Internet Protocol – it even has a complicated name - SIP stands for Session Initiation Protocol. 

But don’t let that put you off. Although the technology is complex, making calls via business VoIP or SIP trunking is as simple as using a business landline or mobile.  

You may not realise, but you’re probably already using VoIP to some degree anyway. If you’ve ever made a call using FaceTime, WhatsApp, Skype or Zoom, then you’ve been connecting over an internet connection instead of a traditional phone line. And that’s all the VoIP is.  

But SIP trunking does everything that VoIP can, and more. Here’s all you need to know. 

What is SIP trunking? 

SIP trunking is a way to make calls using a data connection. It works in much the same way as VoIP, but can also handle video calling and data messaging, alongside voice calling. Calls can be made on a shared business broadband line or on a leased line. 

If you use SIP trunking on a leased line, calls will be made using a dedicated internet connection that’s shared with anyone else. This means less traffic on your line, which will give you faster speeds and more reliable connectivity. It can also handle more than just voice calls, meaning you can also use it for video and text messages. 

How does SIP trunking work? 

To get an idea of how SIP trunking works, you need to think of trunking as in ‘tree’ and not as in ‘elephant’. With traditional phone lines, the trunk refers to a bundle of phone lines that branch off to different users.  

SIP trunking is a virtual version of the analogue phone system. It connects the private branch exchange (PBX) at your business premises to the public switched telephone network (PSTN), so you can communicate with the outside world. 

Is SIP trunking the same as VoIP? 

Although SIP trunking and VoIP are both ways to make calls over an internet connection rather than a phone line, there are differences between the two technologies.  

The main difference is that SIP trunking can handle multimedia data, including voice, text and video, while VoIP is solely for voice calls. 

Another big difference is that while VoIP calls can only be made over an internet connection or private network, SIP trunking can handle multimedia calls over any network, including ISDN, VPN, or the internet. 

What’s the difference between a SIP line and a SIP trunk? 

Although the two terms are sometimes interchangeable, it’s important to know the difference between 

SIP lines and SIP trunks as it could affect the price of your package. Bear the following in mind:

  • A SIP trunk is a collection of lines 
  • A SIP line is simply one call within a SIP trunk 

Or, to go back to the tree analogy, a SIP trunk is the base that connects your business phone service to the outside world, while a SIP line is one of the branches off it. And, just like a tree, a SIP trunk will have many lines (branches) that run off it. 

This is important to remember when choosing a SIP trunk service, as you need to make sure the one you choose has enough capacity to meet your needs. For example: 

  • If you have a call centre that can take up to 50 calls at any one time, then you’ll need a SIP trunk that can handle at least 50 SIP lines 
  • If your call centre can take up to 200 calls at once, you’ll need a SIP trunk with the capacity for at least 200 lines 

The good thing about this technology is that it’s easily scalable, meaning it can grow as your business grows. 

What are the advantages of SIP trunking? 

As with VoIP for business, SIP trunking can offer many benefits, including: 

  • Lower communication costs – SIP trunking uses the internet or an IP network instead of a physical phone line, which means you’ll benefit from cheaper call costs and line rental charges. Domestic and in-network calls are free, and the technology also helps to cut the cost of international calls. 
  • Simple multimedia messaging – Instead of subscribing to different services, SIP trunking allows you to use the same IP-based network for all types of data transmission, such as voice, video, and more. This can save you the hassle of keeping on top of and paying for multiple networks and subscriptions.  
  • Flexible phone numbers – When you use a SIP trunk, you’re no longer tethered to the local exchange. This means you can choose any available local or national number to suit your needs. If you change business premises, you can take your number with you. 
  • Future-proof your business – Traditional analogue phones lines are being phased out before a complete switch off in 2025. Switching to VoIP or SIP trunking now will help to future-proof your business by using technology that keeps it one-step ahead of the competition. 
  • Simple installation and maintenance – A reliable internet connection is all you need to get up and running with a SIP trunk. There’s no need to install or maintain any hardware or physical phone lines. 
  • Scale your system to suit your needs– It's important to have systems that can easily adapt to the size and needs of your business, whether it’s growing or scaling things back. More lines can easily be added or taken away from your SIP trunk, without the need for any work at your premises. 
  • Offer better customer service – SIP trunking allows your business to easily communicate with customers in a variety of ways. You can use voice, video and text services on the same system to help align and improve sales, marketing and customer service. 

How do you set up a SIP trunk? 

A reliable internet connection with good bandwidth and speed is the minimum requirement for setting up a SIP trunk. SIP trunking supports multimedia sessions, including video calling, which will need faster speeds than voice calling with VoIP. 

The speeds and bandwidth you need will be determined by the number of SIP lines you have as part of your trunk. When working out the size of the plan you’ll need, always base it on the maximum number of calls you’ll need to make at any one time. More users need greater bandwidth. 

If you’re switching from an old ISDN network, remember that your SIP trunk can support as many calls as you need, rather than as many calls as your ISDN trunks can handle. 

Once you know how many lines you need and you’re sure you have sufficient speeds and bandwidth to handle things, your SIP trunk will be set up remotely by your provider. The only physical work that may need to be carried out is running an Ethernet cable from your router to your PBX to connect it to your internet connection. 

Once everything is set up, you’ll need to migrate all your phone numbers from ISDN to the SIP platform. This should be handled by your old and new providers as part of the switch. 

How much does it cost to install SIP trunking? 

This will depend upon the number of SIP lines you need as part of your trunk and the provider you go with, but costs should be relatively low as there’s no need for any physical installation or maintenance work at your property. 

To find the right connectivity solution for your business, give our tech-enabled experts a call on 0800 086 1920. Or leave your postcode in the box on the right and we’ll give you a call back.