What Is a SIP Trunk and Do You Need One?

Last Updated
September 4, 2026
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A SIP trunk is a virtual phone line delivered over the internet, connecting an existing on-premise phone system to the wider telephone network without needing physical ISDN or analogue lines. It replaces the traditional copper connections businesses used to rely on, letting calls travel as data over your internet connection instead. You'll typically need one if you already have (or want) your own PBX phone system, such as 3CX, rather than a fully hosted, cloud-based system where this connectivity is built in.

How Does a SIP Trunk Work?

SIP stands for Session Initiation Protocol, the technical standard used to set up, manage, and end voice calls over the internet. A SIP trunk provider gives you a set of channels — each one capable of carrying a single simultaneous call — connected to your phone system through your internet connection, rather than a physical line running into the building.

When someone calls your business number, the call travels over the internet to your SIP trunk provider, who routes it to your on-premise phone system. Outbound calls work the same way in reverse.

What's the Difference Between a SIP Trunk and Hosted VoIP?

This is where a lot of confusion comes from, since both use the internet to carry calls, but they're structured quite differently:

  • SIP trunking connects your own on-premise phone system to the phone network. You still own and manage the PBX hardware or software yourself, such as a 3CX installation, and the SIP trunk simply replaces the physical line into it.
  • Hosted VoIP (like Gamma Horizon) runs the entire phone system in the cloud. There's no on-premise PBX to manage at all — connectivity, call handling, and the phone system itself are all provided as a single service.

If you already have, or specifically want, your own phone system rather than a fully managed cloud platform, SIP trunking is the connectivity layer that makes it work.

Do You Need a SIP Trunk?

You'll generally need a SIP trunk if:

  • You already have an on-premise PBX system, such as 3CX, and are moving off traditional ISDN or analogue lines ahead of the PSTN switch-off
  • You want more control over call routing, number ranges, or carrier choice than a fully hosted system typically offers
  • You're setting up Direct Routing for Microsoft Teams, which relies on a SIP trunk connected through a certified session border controller

You probably don't need one specifically if you're moving to a fully hosted system like Gamma Horizon, since that connectivity is already built into the service — you'd be adding a layer of complexity you don't actually need.

What Are the Benefits of SIP Trunking?

  • Cost — typically cheaper than traditional ISDN lines, and calls often cost less too, particularly for businesses with higher call volumes
  • Flexibility — channels can usually be added or removed more easily than physical lines, scaling with demand
  • Keeping your existing PBX — useful if you've already invested in a system like 3CX and don't want to replace it entirely
  • Business continuity options — calls can often be rerouted quickly to a different location or mobile numbers if your main site loses connectivity, provided this is set up in advance

What Do You Need to Use SIP Trunks?

  • A compatible on-premise phone system, such as 3CX or another SIP-based PBX
  • A reliable business internet connection with enough bandwidth for your expected call volume — see our guide to VoIP bandwidth requirements
  • A SIP trunk provider, who allocates the channels and connects you to the wider phone network
  • Correctly configured Quality of Service (QoS) settings, so voice traffic is prioritised over other network activity

For businesses with more demanding or business-critical needs, a dedicated connection such as a leased line is sometimes worth considering alongside SIP trunking, rather than relying on standard broadband.

Frequently Asked Questions

How many SIP trunk channels does my business need? This depends on your maximum number of simultaneous calls, not your total headcount — our guide to SIP trunks covers sizing in more detail.

Can I use SIP trunking with Microsoft Teams? Yes, through Direct Routing, which connects a SIP trunk to Teams via a certified session border controller. Our guide to Teams Direct Routing explains how this works.

Is SIP trunking suitable for a small business? It can be, particularly if you already have a PBX system in place. For businesses starting from scratch without existing hardware, a fully hosted system is often simpler to set up and manage.

Do I need a SIP trunk if I'm switching away from ISDN before the switch-off? Only if you're keeping your existing on-premise PBX system. If you're replacing the PBX entirely with a hosted system, SIP trunking isn't needed as a separate step.

Whether SIP trunking or a fully hosted system suits you depends on what you've already got in place. Request a quote for independent advice on the right fit, or use our phone system finder to compare your options.