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Glossary · VoIP and call quality

SIP trunk

In one sentence

A SIP trunk is a voice-over-IP connection between a business phone system and a carrier. It replaces ISDN lines and carries calls and numbers over your internet connection.

Also known as : SIP trunks · SIP trunking · SIP line · SIP lines

How a SIP trunk worksA four-step flow. 1: the company's handsets connect to its PBX (3CX, Asterisk, Teams…). 2: the PBX registers with the carrier over a SIP trunk running on the internet connection. 3: the carrier converts the calls and hands them to the public phone network. 4: the call reaches a landline or mobile, showing the company's number.1 · Company handsetsIP phones, softphones,mobile app2 · Company PBX3CX, Asterisk, Teams…sends the call over SIP3 · Carrier (SIP trunk)Authenticates, routes andhands off to the PSTN4 · The person calledLandline or mobile, seesthe company's number
A SIP trunk replaces physical lines: the PBX stays where it is and calls travel over the internet to the carrier.

What is a SIP trunk?

Business phone systems used to connect to the public network through physical lines: ISDN2 for two simultaneous calls, ISDN30 for up to thirty. A SIP trunk does the same job as a logical link over your internet connection or a dedicated carrier circuit. Your PBX stays where it is, on site or hosted by your IT partner, but it sends and receives calls using SIP.

“Trunk” is old telecoms language for a bundle of circuits shared by many users. A 10-channel SIP trunk carries 10 conversations at once, whatever the number of handsets behind it.

How it works

  1. The PBX authenticates with the carrier, either with a username and password (SIP registration) or by its fixed public IP address.
  2. For an outbound call it sends a SIP INVITE with the dialled number and the number to present to the person called.
  3. The carrier checks that the presented number belongs to the customer, picks a route and hands the call to the phone network.
  4. For an inbound call the carrier sends the call down the trunk with the dialled number, which the PBX uses to ring the right extension through DIDs.

The voice itself travels separately as an RTP stream, compressed by a codec such as G.711. When the network is more complex (NAT, several sites, Microsoft Teams), a session border controller acts as the secure crossing point.

Example

A 25-person architecture practice in Manchester runs a 3CX system installed in 2020 on an ISDN30 line with 12 channels in use. With Openreach withdrawing ISDN, the practice orders an eight-channel SIP trunk after checking its call reports: it never had more than six calls at once. Its 0161 numbers and DID range are ported, the trunk is added in the 3CX console, and the ISDN terminal box is removed. Extensions, call flows and the IVR stay exactly as they were.

SIP trunks at Universal Telecom

Our SIP trunk works with the common PBX platforms and lets you port your existing numbers. If you have no PBX, or yours is near the end of its life, a virtual PBX removes the box altogether. Carriers and integrators reselling voice will find their offer on the wholesale page.

Frequently asked questions

Is a SIP trunk the same as VoIP?

A SIP trunk is one way of using VoIP. VoIP is the general technology of carrying voice over IP networks; a SIP trunk is the specific link that connects an on-site or hosted PBX to a carrier using the SIP protocol.

How many SIP channels do I need?

As many as the calls you have running at the same time at your busiest moment, not one per employee. Call reports from your current system give the real peak; many 20-person offices need six to eight channels.

Do I need a SIP trunk before the ISDN switch-off?

In the UK, BT Openreach is retiring ISDN and analogue lines as part of the move to all-IP. If your PBX still uses ISDN2 or ISDN30, a SIP trunk or a cloud PBX is the usual replacement, and your numbers can be ported.

Related terms

  • SIP

    SIP (Session Initiation Protocol) is the standard signalling protocol that sets up, changes and ends VoIP calls. It handles ringing, answering and hanging up; the voice itself travels separately.

  • PBX

    A PBX (private branch exchange) is a business's own telephone switch. It connects internal extensions to each other and to the public network, and runs features such as IVR menus, queues and voicemail.

  • SBC (session border controller)

    A session border controller (SBC) sits at the edge of a network and controls SIP calls going in and out: it filters traffic, hides the internal network, handles NAT and can transcode or encrypt calls.

  • DID (direct inward dialling)

    A DID (direct inward dialling) number, called a DDI in the UK, is a public phone number that rings a specific extension, team or call flow directly, without passing through a switchboard.

  • Codec

    A codec is the algorithm that digitises and compresses your voice during a VoIP call and turns it back into sound at the other end. It sets both the audio quality and the bandwidth used.

  • Number porting

    Number porting is moving an existing phone number from one provider to another so the business keeps the same number. The new provider runs the process; the old service ends when the port completes.

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