Calling Line Identification: how CLI is regulated in the UK
In short: Calling line identification (CLI) is the caller information carried with a telephone call. In the UK it is governed by Ofcom General Condition C6 and by regulations 10 to 13 of PECR. Ofcom’s CLI guidance sets how providers must handle it. A further change, requiring inbound +447 presentation numbers to be withheld, applies from 15 July 2027.
Every phone call shows the recipient a number. Whether that number is accurate, and whether it may be shown at all, is what the calling line identification rules govern. Accurate CLI lets people decide whether to answer and lets a business be called back; a wrong or spoofed number is how many scam calls get someone to pick up. This page sets out how CLI is regulated in the UK, how the rules have developed, and the change communications providers need to prepare for before it applies from 15 July 2027.
What calling line identification is and why it matters
Calling line identification is the data about the caller that is passed between networks and, where permitted, displayed to the person receiving the call. CLI data has two parts: a telephone number identifying the line, and a privacy marking indicating whether the number may be shown to the recipient. Accurate CLI lets people judge whether to pick up, and lets providers and enforcement bodies trace nuisance and scam traffic. The same mechanism can be abused. Spoofing, where false information is inserted to mislead the recipient about who is calling, is the harm the CLI rules are designed to reduce, while still protecting the right of an individual caller to withhold their number.
The legal framework: General Condition C6 and PECR
The core obligation is set out in Ofcom’s General Condition C6, made under the Communications Act 2003. C6 requires communications providers, where technically feasible and economically viable, to provide CLI facilities, and to ensure that any CLI data provided with a call includes a valid, dialable telephone number that uniquely identifies the caller. General Condition C6.6 adds a further duty on accuracy: where technically feasible, providers must take all reasonable steps to identify calls whose CLI data is invalid, does not uniquely identify the caller, or does not contain a dialable number, and prevent those calls from being connected. Calls to the emergency services are the exception and must always be connected, whatever the state of the CLI.
The privacy rules apply alongside the telecoms rules. CLI data is personal data, so providers must comply with the UK GDPR, the Data Protection Act 2018 and the Privacy and Electronic Communications (EC Directive) Regulations 2003 (PECR). Regulations 10 to 13 of PECR confer the main CLI privacy rights: a caller’s right to withhold their number (regulation 10), a called party’s right to be informed of an incoming number (regulation 11), the right to reject calls from lines where the number is withheld (regulation 12), and connected line identification (regulation 13). A separate provision applies to marketing calls: the Privacy and Electronic Communications (EC Directive) (Amendment) Regulations 2016 inserted regulation 21(A1) into PECR, which requires anyone making direct marketing calls either not to prevent presentation of their number or to present a number on which they can be contacted.
Network Number and Presentation Number
The guidance rests on a distinction between two elements of CLI data, and most of the rules that follow depend on it. The Network Number identifies the point at which the call enters the network. It must be present on every call and must be a valid, dialable number. Where a call is connected despite a missing or invalid Network Number, for example while the provider investigates the cause, the provider should insert a number from a range allocated to it for that purpose and mark it unavailable so that it is not displayed. The Presentation Number is the number shown to the person being called. It may be the same as the Network Number, or a different number where the caller has a legitimate reason, such as a switchboard presenting its main line for outbound calls. Where it differs, the Presentation Number must still be a valid, dialable number that the caller is entitled to use.
What providers must do across the call path
Every provider in the chain is responsible for accurate CLI, because a single call often crosses several networks. The originating provider must ensure the CLI is correct at the point of origination, and that a customer using a different Presentation Number is entitled to do so. Transit providers must not strip, alter or replace CLI data except in defined circumstances, and where technically feasible should check that the CLI they pass is valid and dialable, for example by confirming that a UK number comes from a range allocated in the National Numbering Scheme. The terminating provider must present only valid, dialable CLI to the end user and block calls with invalid CLI before connection. For calls originating abroad, the check falls on the provider at the first point of ingress to the UK network; a provider receiving a call from an international gateway is expected to satisfy itself, usually by contract, that the gateway has carried out the relevant checks. Ofcom enforces these obligations under the Communications Act 2003, with financial penalties available for breach of the General Conditions.
How the CLI guidance has developed
Ofcom has revised its CLI guidance repeatedly as spoofing techniques have changed. The current text builds on guidelines first published in 2003 and amended in 2007, then replaced by the 2018 guidance. Changes published in November 2022 improved the reliability of CLI data and reflected amendments to General Condition C6.6, including which numbers can be used as a CLI and how international calls with invalid numbers are handled. Under the version in force since 29 January 2025, published on 29 July 2024, Ofcom expects providers to identify and block calls from abroad that use a UK number as a Presentation Number, except in a limited set of legitimate cases. The most recent revision was published on 15 July 2026 and applies from 15 July 2027. The revisions are consistent. Ofcom has steadily reduced the scope for a call from outside the UK to arrive with a UK number that misleads the recipient.
The change coming into force: +447 presentation numbers from 15 July 2027
From 15 July 2027, the guidance requires providers to withhold UK mobile numbers presented on inbound international calls. The change, part of Ofcom’s wider work to block more calls with spoofed numbers, adds a new paragraph, 4.18A, dealing with calls that enter a UK network from abroad carrying a CLI from the +447 (UK mobile) range. At the first point of ingress, the provider must modify the call to stop that number being shown as a Presentation Number, by changing the privacy marking so the Presentation Number is treated as withheld. The call is not blocked outright; the misleading UK mobile number is simply not displayed. This extends to +447 mobile ranges the approach Ofcom already applies to other UK numbers arriving from abroad. A call from outside the UK or the Crown Dependencies should not use a UK number as its Network Number or Presentation Number except in defined cases: calls to a mobile user roaming in the UK, traffic the provider can show originated on a UK network before being routed abroad and back, and calls the provider can show are made by a UK customer whose traffic originated on a non-UK network. For +447 numbers a provider can also rely on home-routing calls from its own customers roaming abroad. Outside those cases, calls from abroad using a UK CLI should be blocked. The 2026 revision also updates references to the relevant technical standards.
| Theme | Position to 15 July 2027 (29 July 2024 version) | Position from 15 July 2027 (15 July 2026 version) |
|---|---|---|
| Inbound +447 presentation numbers | No mobile-specific rule; general expectation that calls from abroad do not use a UK number as a Presentation Number save in limited cases. | New paragraph 4.18A: inbound +447 CLI must be modified at first point of UK ingress to withhold the Presentation Number. |
| Method of treatment | Identify and block calls from abroad using a UK Presentation Number outside the permitted cases. | Change the privacy marking so the +447 number is not displayed; the call itself is not blocked. |
| Legitimate-use exceptions | Roaming, UK-originated onward-routed traffic, and UK-customer traffic originating abroad. | Same exceptions, plus reliance on home-routing for a provider’s own roaming customers on +447 numbers. |
| Technical standards | References as set in the 2022 and 2024 revisions. | References to the relevant technical standards updated. |
| Commencement | Applies from 29 January 2025. | Applies from 15 July 2027. |
What this means for communications providers
For providers and their business customers, the CLI rules come down to a few concrete disciplines. Customer terms should require accurate use of CLI and evidence of authority for any Presentation Number that differs from the Network Number. Signalling arrangements, and the wholesale agreements behind them, should not strip or replace CLI parameters except where the guidance permits. Providers should run automated CLI validation at ingress against the National Numbering Scheme and the industry Do Not Originate list, and providers carrying inbound international traffic should plan for the +447 privacy-marking change well ahead of 15 July 2027. Providers reduce their exposure if Ofcom asks questions by keeping a written call-blocking policy and an audit trail of blocked calls, and by running a complaints process that preserves evidence. Providers unsure whether they fall within General Condition C6 at all should start with the scope question, which depends on whether they provide a number-based interpersonal communications service or a network over which one is provided. Our page on numbering and the wider telecoms regulation practice area set out how these obligations fit together, and our complaints and investigations page covers what an Ofcom enquiry looks like in practice.
Frequently asked questions
What is calling line identification?
Calling line identification is the caller data carried with a telephone call: a telephone number identifying the line, and a privacy marking indicating whether the number may be shown to the recipient. It underpins caller display and call-backs and lets providers trace nuisance and scam calls; in the UK it is regulated under Ofcom General Condition C6 and PECR.
Which rules govern CLI in the UK?
Ofcom General Condition C6, made under the Communications Act 2003, requires providers to offer CLI facilities and to ensure CLI data is a valid, dialable number that uniquely identifies the caller. Regulations 10 to 13 of PECR confer the privacy rights of callers and called parties, and Ofcom’s CLI guidance explains how providers should meet these obligations across the call path.
What changes on 15 July 2027?
From 15 July 2027, new paragraph 4.18A of the CLI guidance requires providers to modify inbound international calls carrying a +447 (UK mobile) number, at the first point of UK ingress, so that the number is not displayed as a Presentation Number. This is done by changing the privacy marking to withheld rather than blocking the call, and it extends the existing treatment of UK numbers arriving from abroad to mobile ranges.
What is the difference between a Network Number and a Presentation Number?
The Network Number identifies the point where the call enters the network and must be present on every call as a valid, dialable number. The Presentation Number is the number shown to the recipient. It may match the Network Number or differ where the caller is entitled to use another number, such as a switchboard presenting its main line, but it must still be valid and dialable.
For advice on whether General Condition C6 applies to your service, or on preparing for the +447 change before it takes effect, contact Rob Bratby at Bratby Law. If you are launching or changing a telecoms product, our direct legal advice page sets out when it is worth taking a view early.
