Honestly, SIP trunk pricing in the UK is one of the most confused conversations I have with customers. Let me untangle it properly because the pricing models are not interchangeable and the wrong choice can easily double your monthly bill.
The two models that actually exist
Per-channel pricing
You pay for each concurrent call channel you reserve. 10 channels = 10 simultaneous calls possible at any moment. Typical UK rate in 2026 is £3-7 per channel per month.
Example, a 25-person office with 8 concurrent calls at peak buys 10 channels, pays £40-70/month for the trunk, plus per-minute call charges.
Concurrent call bursting (sometimes 'dynamic' or 'pay-as-you-go')
You pay a base for capacity but can burst above it at premium rates. 10 base channels + unlimited burst at £0.02/min extra per burst channel.
Example, same 25-person office. Pays £40 for 10 base channels, bursts to 12 occasionally, that hour costs extra £0.02 × 2 × 60 = £2.40 for the burst.
The hidden third model — per-minute only
Some UK providers quote 'just pay per minute, no channel fees.' Sounds attractive. The fine print typically says minutes are billed at 2-3x the standard rate to cover infrastructure. If your monthly minutes exceed 2,000, this model always works out more expensive.
Which model fits which business
| Business type | Minutes/month | Best model |
|---|---|---|
| Small office, predictable volume | <1,500 | Per-channel fixed |
| Seasonal / hospitality / retail | Spikes 3-5x normal | Concurrent bursting |
| Contact centre | 10k+, predictable | Per-channel fixed with volume discount |
| Startup, unpredictable growth | Unknown | Per-channel, room to grow |
| Testing / dev use | Tiny, irregular | Per-minute only (rare valid use case) |
From my experience at DialPhone supporting about 180 UK businesses on SIP trunks since 2023, per-channel fixed pricing is the right answer for maybe 85% of customers. The rest genuinely benefit from bursting.
The channel-count question
This is where people get it most wrong. How many channels do you need?
The wrong way to estimate
Count employees. 25 employees = 25 channels. This almost always over-provisions. In reality only 15-40% of employees are ever on a call simultaneously.
The right way
Two approaches, both defensible.
Method 1, 70% of peak concurrent calls
Look at your current PBX concurrent-call reports. Take the peak hour across the last 30 days. Multiply by 1.3 for buffer. That is your channel count.
Example, Leeds estate agency, 22 employees, peak concurrent was 8 calls at 10:15am Monday. 8 × 1.3 = 11 channels. Buy 12 rounded up.
Method 2, Erlang C calculation
Formal method used by contact centres. Inputs are calls per hour, average handle time, target blocking probability. Outputs minimum channels needed to hit blocking under 0.1%.
Online Erlang calculators exist (free). For an SMB this is overkill, Method 1 is enough.
The thing providers will not tell you on the first call
Many UK SIP providers oversubscribe their upstream capacity. They might sell you 10 channels but only have infrastructure to handle 7 concurrent if all customers burst at once. During peak events this causes call failures.
Ask every provider you evaluate, 'what is your oversubscription ratio' — if they cannot answer, assume it is bad. Good providers run at 1:1 (what they sell is what they have). Lazy providers run 3:1 or worse.
Hidden fees to watch
- Emergency services fee — some providers charge £0.50-2 per channel per month for 999 provisioning. Technically required by Ofcom. Some providers absorb this, some pass it through.
- Number translation fees — receiving calls to 0800 numbers may cost you £0.02-0.05 per minute even though 0800 is 'free'. Free for the caller, not for you.
- International destinations — some UK providers charge 10x inflated rates for calls to 'unusual' destinations like Vatican City or the Falklands. If you call any unusual destinations, check first.
- Porting fees — reputable providers do not charge. Those that do typically charge £10-50 per number ported in/out. Negotiate it to zero or walk.
What we charge at DialPhone for comparison
| Product | Price |
|---|---|
| Per-channel SIP trunk (fixed) | £4.50/channel/month |
| Minute bundle 1,000 mins UK local | £18/month |
| Minute bundle 1,000 mins UK mobile | £42/month |
| Per-minute out of bundle, UK local | £0.008 |
| Per-minute out of bundle, UK mobile | £0.042 |
| 999 provisioning | Included |
| Porting | Included |
| Burst channels | £0.02/min/channel extra |
We are roughly middle of the UK market. The cheapest legitimate providers charge around £3/channel, the most expensive up to £9. Anything at £2 or below is cutting corners on something — look hard at their SLAs, call quality reports, and support response.
The honest advice if you are comparing SIP providers today
- Work out your real concurrent channel need (Method 1 above, not employees × 1)
- Get 3 quotes on the same channel count and minute bundle
- Ask each about oversubscription and hidden fees
- Pick the middle price, not the cheapest
The cheapest UK SIP trunks almost always have a catch. Middle-priced providers tend to be honest about infrastructure, and the expensive ones are usually incumbent carriers who know you will not switch.
If you want a second opinion on a specific quote, I am happy to look at it. No pitch, just honest analysis. DialPhone.
