Most businesses don't notice how much they spend on telephony until something forces a review. A line contract comes up for renewal, an ISDN retirement notice arrives, or the finance team asks why call costs keep climbing.
If that sounds familiar, comparing SIP trunk providers in the UK is a sensible next step. But the cheapest quote isn't always the cheapest outcome. This article shows where phone costs hide, how pricing works and how to choose a provider you won't regret in a year.
Where Call Costs Quietly Build Up
Traditional ISDN setups tend to cost more than they appear to, because the charges are spread across several lines on the bill:
Line rental for each ISDN channel, whether it is in use or not.
Call charges, often higher for mobile and international destinations.
Hardware maintenance on ageing PBX equipment and gateways.
Separate lines and contracts at each office.
Because ISDN is sold in fixed blocks of channels, many firms pay for capacity they only need at peak times. A 12-person office on an ISDN2e line may use far fewer channels most of the day, yet pays for them all.
With the PSTN and ISDN switch-off scheduled for January 2027, staying put is also becoming a risk. Moving is less about saving money and more about staying connected.
What a SIP Trunk Replaces
A SIP trunk carries voice calls over an internet connection instead of copper lines. It links your phone system to the public network, so you keep your UK numbers and make calls as before.
You'll sometimes see this called an IP SIP trunk, meaning calls run over IP rather than circuit-switched channels. It works with an on-site PBX or a cloud PBX.
It isn't the same as hosted telephony. With hosted telephony, the provider runs the whole phone system. With a SIP trunk for business, you keep your own system and the provider handles call routing and numbers. That makes it a good fit for companies with equipment they don't want to replace yet.
How SIP Pricing Works
Pricing models vary, and comparing them like for like is where most buyers go wrong.
Per-Channel Pricing
You pay a monthly fee for each concurrent channel (one channel handles one call at a time), plus call charges. This suits businesses with predictable call volumes and gives clear control over capacity.
Bundled Minutes
Some providers include a pool of minutes. This can work well, but check whether mobile calls are included. Many bundles cover UK landlines only, and mobile calls are billed separately.
Per-User Plans
Common with hosted services, these charge per person rather than per channel. They are simple to budget for, but can cost more for businesses where many staff rarely make calls.
Costs Often Missed in Quotes
Ask about these before comparing totals:
Setup and number porting fees.
Charges per phone number or DDI range.
Minimum contract lengths and early exit costs.
Call charges to international and non-geographic numbers.
Extras such as failover, call recording or fraud protection.
A quote that looks low can end up higher once these are added.
Cheapest or Best? Think in Total Cost
Searches for the cheapest UK VoIP providers are natural, but price per minute is only part of the picture. A provider with weak support or no failover can cost far more in downtime than it saves.
Consider a trade business. If its phone line fails for half a day, it may lose jobs it never knew it missed. A slightly higher monthly fee for automatic failover to a mobile can pay for itself in a single incident.
A better approach is to compare total cost of ownership across setup, monthly fees, call charges, support and risk. The best UK VoIP providers for your business are those that fit your call patterns, existing equipment and tolerance for downtime.
What to Check Beyond Price
Once you have a rough price range, narrow the list using these criteria. They separate dependable UK SIP trunk providers from those that look good only on paper.
Call Quality
Voice is sensitive to delay, jitter and packet loss. Ask how the provider routes calls and whether Quality of Service (QoS) or private connectivity is available. As a rough guide, a G.711 call needs around 80 to 100 kbps in each direction once overheads are included, so your broadband or leased line needs to cope at peak times.
Resilience
Find out what happens if your connection or the provider's network fails. Good providers can reroute calls to a mobile, another site or a backup route. Ask whether their infrastructure is geographically separate and whether failover is included.
Number Porting
Check that the provider can port your geographic (01/02), non-geographic (03, 08) and DDI numbers, and ask for typical timescales. Delays here are one of the most common reasons migrations stall.
UK Compliance
Confirm how 999 and 112 calls are handled, including location information. Ask whether caller line identification (CLI) follows Ofcom rules, and where call recordings and data are stored.
Phone System Compatibility
Make sure your platform, whether Avaya, Mitel, Cisco, 3CX or FreePBX, is supported. Older systems may need a gateway or session border controller, which should be factored into the cost.
Examples from Real Business Setups
An accountancy firm with 15 staff might replace its ISDN2e line with four to six SIP channels, keep its numbers and add mobile failover. Staff can take calls on softphones during peak deadlines, and line rental disappears.
A dental or estate agency group with several branches can run one central trunk with separate DDI ranges for each site. Internal calls between branches no longer need dedicated lines, and call management becomes simpler.
A growing sales team can add channels ahead of a busy season and remove them afterwards, rather than paying for fixed lines all year.
Questions Worth Asking Each Provider
A short conversation can reveal a lot. Ask:
How long does porting usually take, and who manages it?
What uptime do you commit to, and what happens if you miss it?
Is UK-based support included in the standard price?
How quickly can channels be added or removed?
When building a shortlist, include specialists alongside the national carriers. A provider such as Wavetel Business may offer more hands-on help during migration than a large operator, which matters if you don't have an in-house IT team. Whoever you consider, ask for references from companies of a similar size.
FAQs
What is the difference between a SIP trunk and VoIP?
VoIP is the broad technology of sending voice over the internet. A SIP trunk is one application of it, connecting your business phone system to the public network. A hosted VoIP service usually includes the phone system itself, while a SIP trunk doesn't.
Is SIP cheaper than ISDN?
Often, but not always. SIP removes line rental and lets you pay only for the channels you need. Savings depend on your call volumes, contract terms and extras such as failover, so compare total cost rather than headline rates.
How many SIP channels do I need?
One channel handles one call at a time. Count your typical peak number of simultaneous calls, then add a small margin. Many offices need fewer channels than they expect, since not everyone is on the phone at once.
Can I keep my existing business numbers?
In most cases, yes. UK numbers can be ported, though timescales vary by number type and current provider. Confirm porting before cancelling any existing service.
Conclusion
Overpaying for calls usually comes down to paying for capacity you don't use, or ignoring costs that sit outside the headline rate. A SIP trunk service can fix both, as long as the provider is reliable and the pricing is transparent.
Start with a simple audit: how many calls you make at peak, which numbers must stay and whether your internet connection can handle voice. Then shortlist two or three providers, ask for full pricing including setup and porting, and test call quality before signing a long contract. With the ISDN switch-off close, acting soon will give you more choice and less pressure.
