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SIP trunking delivers your calls over your internet connection instead of copper, connects into the phone system you already own, and keeps your existing numbers. For a business that spent real money on a phone system in the last few years, it is usually the cheaper way through the switch-off, and it is the option most switch-off guides leave out entirely.
This guide covers what a SIP trunk is, what one costs in the UK, how many you need, and what to check before you sign anything.
What is a SIP trunk?
A SIP trunk is a phone line delivered over the internet. SIP stands for Session Initiation Protocol, which is just the standard used to start and end a call across an IP network.
An analogue line carried one call over a pair of copper wires. An ISDN circuit carried a fixed number of channels over a physical connection into your building.
A SIP trunk does the same job; it carries calls into your phone system, but it arrives over your broadband or leased line rather than the cable in the ground.
That software point is the one that matters day to day. Adding ten more channels to a SIP trunk is a configuration change made in hours. Adding ten more channels to an ISDN circuit meant an engineer, a lead time and a bigger bill every month afterwards.
SIP trunking vs hosted VoIP: which one does your business need?
Both replace copper. The difference is what happens to your phone system.
With hosted VoIP, the phone system itself moves to the cloud. Your provider runs it, you pay per user per month, and your handsets connect over the internet. Nothing sits on site except the phones.
With SIP trunking, the phone system stays where it is. Only the lines feeding it change.
The decision usually comes down to one question: do you own a phone system that still does what you need?
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Best for |
Businesses with a working on-site phone system |
Businesses with no system, or one near end of life |
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What you keep |
Your phone system, handsets and numbers |
Your numbers |
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Priced by |
Channel, one per simultaneous call |
User |
|
Upfront cost |
Low, usually configuration only |
Handsets, if yours are not IP |
|
Who maintains it |
You or your IT support |
Your provider |
|
Scaling up |
Add channels in software |
Add users in software |
As a rule of thumb, if your phone system is out of manufacturer support or coming up to a decade old, hosted VoIP is the better answer; you will be replacing it soon regardless, and the switch-off is a sensible moment to do it. If it was installed recently and does everything you ask of it, SIP trunking lets you keep it and still meet the deadline.
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What does a SIP trunk cost?
SIP trunks are priced per channel, per month. Across the UK market, a channel typically runs between £2.50 and £6.00 a month, and most providers discount at volume; at ten channels or more, £3.00 or less per channel is common.
Channel rental is not the whole bill, though. These are the other lines to expect on a quote:
|
Channel rental |
£2.50 – £6.00 per channel, per month |
|
Setup |
£0 – £50, often waived |
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Number porting |
£0 – £10 per number, frequently free |
|
Direct dial (DDI) numbers |
£0.50 – £1.50 per number, per month |
|
UK landline calls |
0.5p – 1.5p per minute, or bundled |
|
UK mobile calls |
2p – 5p per minute, or bundled |
Bundled packages that include call minutes tend to land around £4 to £6 per channel per month all in.
Now set that against what copper costs today. Openreach raised the wholesale price of a legacy line three times during 2026: 20% in April, 40% in July and a further 40% on 1 October, leaving the same line at roughly double its 2025 price, with providers passing those rises on. A business still paying for eight copper channels in late 2026 is paying considerably more than it would for eight SIP channels that also do more.
Exact pricing depends on your provider, your contract length and your call profile.
How many channels does your business need?
One channel carries one call. If ten people need to be on external calls at the same moment, you need ten channels.
It has nothing to do with how many phones or numbers you have. You can run fifty extensions and fifty direct dial numbers across eight channels, as long as no more than eight people are ever on a call at once.
- A standard office with mixed roles: roughly one channel for every three to four staff
- A sales, support or bookings team where most people are on the phone: around one channel for every two
- A single site with a handful of staff, such as a shop, a trade business or a small practice: two to four channels is usually plenty
Those are starting points rather than answers. Before you buy, ask your current provider for a call report showing your concurrent call peaks across the last three months. That figure, plus a little headroom, is your channel count.
Getting it slightly wrong is not expensive. Because channels are software, capacity can be added in days rather than the weeks an extra physical line used to take.
Choosing a SIP trunk provider: what to check
Advertised starting prices are rarely the monthly bill. When looking for a SIP trunking provider, there are a few things to check to make sure it's the right plan for your business.
- Number porting: Check that every number you want to keep can be ported, and get the timescale in writing. A single number usually takes five to ten working days; a full number range can take ten to twenty-five. Start it early; porting is the step that delays switches.
- A fraud cap: A SIP trunk left unprotected can be used to place expensive international calls, and the bill lands on you. Ask for a spend cap, and out-of-hours call limits set as standard, not as an optional extra.
- What is actually in the call bundle? Confirm whether UK mobile minutes are included, what happens when you exceed the bundle, and the per-minute rate for anything outside it.
- Contract length and renewal terms: Ask what the price does at renewal and how much notice you have to give. Thirty-day terms exist in this market, so a long tie-in should be buying you something.
- Location data: Ofcom requires providers to maintain access to emergency services and to keep the location information behind a 999 call accurate; it ran a compliance programme through to June 2026 checking exactly that. Ask how your emergency location is registered, what happens when you move premises, and how calls route if your internet connection drops.
SIP trunking and the PSTN switch-off
The copper network switches off on 31 January 2027, and Openreach has confirmed there is no extension. As of July 2026, around 1.5 million lines were still running on the old network, roughly 350,000 of them at business premises according to Ofcom data.
If your phone system is fed by ISDN or analogue lines, those lines stop working. The system itself does not necessarily go with them — a phone system with an IP-capable card, or a small gateway sitting in front of it, will carry on running perfectly well on SIP.
The practical order of work is short:
- Ask whoever maintains your phone system whether it is SIP-capable. They will usually know within minutes.
- Get a call report and size your channels from real peaks, not guesses.
- Start the port, and leave a margin.
That last point is the one businesses underestimate. Porting is measured in weeks, and there are 350,000 business premises still to move before the deadline.
SIP trunking FAQs
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Is SIP trunking the same as VoIP?
Not quite. VoIP is the broad term for carrying calls over the internet. SIP trunking is one form of it; the one that feeds your existing phone system rather than replacing it.
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Can I keep my existing phone numbers?
Yes. Geographic numbers, non-geographic numbers and direct dials can all be ported to a SIP trunk. A single number usually takes five to ten working days; a full range takes ten to twenty-five.
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Do I need a new phone system for SIP trunking?
No, provided your current system is SIP-capable or can be made so with a gateway. That is the whole point of SIP trunking. If your system is out of support, hosted VoIP is usually the better route.
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How much does a SIP trunk cost per month?
Typically £2.50 to £6.00 per channel per month in the UK, with discounts for ten channels or more. Bundles including call minutes usually land around £4 to £6 per channel.
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What happens to SIP calls in a power cut?
They stop because the connection runs through equipment at your premises. Battery backup, automatic call divert to a mobile, or a router with 4G failover all cover it; set the divert up before you need it.
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How long does it take to switch to SIP?
The trunk itself can be provisioned quickly. Number porting sets the real timeline; allow several weeks, and longer if you are moving a full number range.
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Is it too late to switch before the 2027 deadline?
No, but the margin is narrowing. The deadline is 31 January 2027 with no extension, and around 350,000 business premises are still to be migrated. Start the port now rather than in December.
