VoipTower's UK SIP connectivity runs verified-CLI only — numbers purchased through VoipTower or verified ported-in — which sits inside Ofcom's legitimate-use carve-outs under the anti-spoofing rules in force since 29 January 2025, ahead of the UK's January 2027 PSTN switch-off. That single design choice is a useful way into a broader question that matters more than it looks: for a company based outside the UK, routing calls into and through UK numbers, what should actually decide which SIP provider gets the contract? Since January 2025, the honest answer is not the price list. It is how the provider's CLI presentation model sits inside Ofcom's rules.

What changed on 29 January 2025

Ofcom's updated CLI rules, set out in its CLI Guidance published 29 July 2024 under General Condition Part C6, took effect on 29 January 2025. From that date, UK telecom providers must block international calls that present a UK Calling Line Identification by default. This is an anti-spoofing measure: it targets the pattern where a call originating outside the UK arrives dressed as a UK number, a technique associated with scam and nuisance calling. The rule carries carve-outs for legitimate cases, among them UK residents roaming abroad and calling home with their own number showing. But the default posture flipped: a UK-presenting CLI on an international leg is now blocked unless it can be placed inside a recognized legitimate-use category.

For anyone routing business traffic with a UK CLI from outside the UK, this is not a theoretical compliance note. It is the mechanism that decides whether a call reaches the person being dialed at all.

The legitimate-use carve-outs

The carve-outs exist because not every international call presenting a UK number is spoofing. Ofcom's framework distinguishes between a number used fraudulently and a number used legitimately by whoever it is actually allocated to, even when the call itself originates abroad. The distinguishing question is provenance: is the CLI tied to a real allocation, whether bought directly or ported in and verified, or is it simply asserted by whoever happens to be dialing?

A provider whose CLI presentation is built around verified ownership, presenting a UK number on a call only when that number was purchased through the provider or ported in and confirmed, sits inside the carve-out logic by construction. A provider that lets any customer assert any UK-looking CLI on outbound traffic sits closer to the pattern the rule was written to stop, regardless of whether any individual instance is actually fraudulent.

It is worth noting what the framework does not yet require: there is no UK STIR/SHAKEN mandate as of 2026. Ofcom consulted on it in 2023 and published an assessment and roadmap in 2024, but implementation is tied to the wider PSTN transition rather than being in force today.

The January 2025 rule was not an isolated move either. On 22 April 2025, Ofcom amended the General Conditions again: tightening B1 provisions around misuse of Global Titles, and extending a new Non-Provider Condition to operators holding UK mobile numbers. Read alongside the CLI rule, the pattern is consistent. Ofcom is closing off routes that let a number be presented or used by someone other than its verified holder, across more than one part of the network, not addressing the anti-spoofing case in isolation.

Why this is a selection criterion, not a footnote

For an international company evaluating UK connectivity, the practical weight of the CLI rule is easy to underestimate because it never shows up as a line item. It shows up as whether calls connect.

It also interacts with a fact that surprises some buyers: getting a UK Geographic DID does not require proof of UK presence. Standard KYC covers the basics: legal name and registration details, a website, a contact person, and a registered address in any country. None of it requires UK incorporation, a UK office, or a UK director. Presence was never really the gate, which means the CLI compliance model is where the real difference between providers actually lives, and it is where due diligence should concentrate.

The UK B2B SIP market has several established providers, and comparing their published rate cards is not hard. Comparing how each one's CLI provenance model maps onto Ofcom's carve-out categories is harder, and it is the comparison that determines whether the numbers provisioned keep working on international routes.

Numbering reality for B2B lines

A short numbering primer matters here because not every UK number type behaves the same way under the CLI rule or fits a B2B deployment equally well. Geographic numbers (01/02 ranges, covering areas such as London, Manchester, Birmingham, Glasgow, Leeds, Liverpool and Edinburgh) are the standard for a local-presence business line. The 03 range is non-geographic but charged at the same rate as a local call, an arrangement distinctive to the UK that does not map cleanly onto other countries' numbering plans. 0800/0808 cover freephone. The 084x/087x premium-rate ranges have seen declining use since Ofcom's 2015 reforms and, notably, cannot be used as an outbound CLI at all. Newer non-geographic ranges (0345/0370) tend to sit with larger corporate deployments, and the 07 mobile range belongs to mobile network operators rather than standard B2B DID provisioning.

None of this changes the CLI compliance picture directly, but it shapes which number type a given use case should be requesting before the compliance question is even asked.

The premium-rate ranges also changed hands administratively on 1 February 2025: the Phone-paid Services Authority stopped acting as the regulator for premium-rate services, and Ofcom took that responsibility directly under a new order, with references to the old regulator removed from the General Conditions. For a B2B buyer the practical takeaway is simple: any premium-rate exposure tied to a UK number is now an Ofcom question end to end, not something split across two regulators.

One more deadline: the PSTN switch-off

Behind all of this sits a fixed date: BT is switching off the UK's PSTN and ISDN network by January 2027, with an estimated 2.4 million UK businesses still needing to migrate to SIP.

The carve-out-fit test

Given all this, the practical question to put to any UK SIP provider is not "what does it cost per channel." It is: does the CLI presented on the calls sit inside Ofcom's legitimate-use carve-outs, and how is that kept true on every call, not just the first one?

That breaks into a few concrete questions worth asking directly:

  • Is the UK number the calls will present one that was actually purchased through the provider or ported in and verified, rather than assigned freely at dial time?
  • What happens to a call if the CLI cannot be matched to a verified allocation, and does the provider actually know the answer or only assume one?
  • Which of Ofcom's legitimate-use carve-out categories does the delivery model fall under, specifically, rather than a general assurance that "compliance is handled"?

A provider that can answer the second and third questions with specifics, not just a claim of compliance in the abstract, is one whose model was built around the post-January-2025 rule rather than retrofitted to survive it. For a business whose UK-direction calls need to land, that is a more useful filter than a rate sheet.

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