What Is An SIP Trunk And How Does It Work?

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If you’ve been researching business telecoms solutions for your company, you’ve probably heard SIP Trunks mentioned a few times. If you’re wondering what they are and how they work this guide will let you know. We hope this helps you decided whether an SIP trunk could be the right choice for your business.

What Is An SIP Trunk?

If you’ve got to the point of looking up an article about SIP Trunks, we presume you already have or are considering a VoIP phone system. SIP is just one of the protocols which can be used on a VoIP system.

SIP is an acronym for Session Initiation Protocol. It refers to a way of initiating and ending connections over the internet for data and voice transfers. In the analogue domain, a trunk used to refer to a bundle of phone lines shared by users.  In digital space, trunking refers to a link between your PBX and the regular telephone network (the PSTN).

A business with a PBX (public branch exchange) needs a way of connecting their internal system to the wider PSTN network. This would previously have been done using ISDN, but with ISDN being phased out, businesses need a new method. SIP Trunks are another way of doing this.

I’ve Heard People Talk About SIP Trunks and SIP Channels. Are These The Same Thing?

No. ‘Trunks’ refer to conventional groups of cables that connect one exchange to another. Through the internet, this process has been virtualised. A trunk is now a grouping of allocated “channels”, each of these channels can handle a set number of simultaneous calls. As a business, the number of channels you require will be determined by the assumed number of simultaneous calls you will need to make and receive at any one time. Each channel can have one inbound and one outbound call running at any one time.

If you are unsure of how many channels you would need, a business telecoms provider can help you calculate this based on your current and predicted usage.

One of the great things about SIP trunks is that they are highly scalable. Unlike analogue phone lines which would require digging up the road to install new lines, sip trunk setup are digital and new channels can be added or removed as and when your business requires them.

What Do I Need To Consider When Choosing A Provider For SIP Trunks?

There are several factors to consider, but if you already have a business telecoms provider who you are satisfied with – they will be able to provide them for you. If you are choosing a provider for the first time, consider the following:



    Pricing for SIP Trunks can differ enormously between suppliers. It’s very important to note that it’s not just about the expense of channels. You will need to consider the call tariffs of the service, as well as the setup costs quoted by the provider, whether you need to move (or “port”) existing numbers and what the costs for repairs or maintenance are.

All reputable enterprise telecom suppliers should be upfront about these costs before you sign any contracts.

There can also be saving to be made by combining several services into a single telecoms bundle. Many business telecoms providers also offer managed IT services, business mobiles or managed print services. You could save a large amount by combining these services as well as having a single point of contact for all your business IT needs.


If your calls are sensitive (such as for financial, healthcare or legal services). Be sure that your provider has a secure connection and data policy? Ideally this will be covered in your service level agreement.


Ensure that your chosen provider has a solid service level agreement (SLA) which includes an agreeable minimum amount of uptime, a suitable response time for engineer callouts (if anything stops working) and an acceptable compensation if this service level agreement is broken.


    No one really knows what the customer service of a business is like until they are one of their customers. The next best thing is to contact existing customers in similar businesses and ask their experience of their business telecoms provider. Use your existing business contacts as well as channel such as LinkedIn to canvas other business owners for their advice. If you find a provider who has overwhelmingly positive review and who people are eager to recommend, it is a good sign that they will be a good choice for a business telecoms provider.

We hope this has given you an effective introduction to the concept of SIP trunks as well as some tips for finding a good business telecoms provider from whom you can obtain them.

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