TL;DR
- UK number porting must complete within one business day under Ofcom rules introduced in 2019.
- Common failures include delays beyond the agreed date, both SIMs briefly active simultaneously, and — rarely — the number being lost entirely.
- The gaining operator (the one you’re moving to) is primarily responsible for managing a successful port.
- Persistent failures should be escalated formally; after eight weeks or a deadlock letter, CISAS or the Communications Ombudsman can adjudicate.
- Keep a written record of your PAC code, the agreed port date, and all contacts with both operators throughout the process.
How UK number porting is supposed to work
Number portability allows a UK mobile customer to keep their existing number when switching to a different operator. The process is regulated by Ofcom under the General Conditions of Entitlement. Since July 2019, Ofcom’s updated porting rules require the entire process — from requesting a Porting Authorisation Code (PAC) to completing the switch — to be faster and simpler than it was previously. A customer can obtain a PAC by texting “PAC” to 65075 from their current number; the code is delivered within 60 seconds by text and email. The PAC is valid for 30 days.
Once the customer provides the PAC to the gaining operator, the port must complete on the agreed date, which must be no later than one business day after the gaining operator submits the port request. The old SIM stops working and the new SIM becomes active, usually early in the morning of the agreed date. The process involves both the losing operator (the one you are leaving) and the gaining operator, who coordinates the technical handover through an industry-managed porting database.
Port not completed on time
The most common porting problem is a delay beyond the agreed completion date. This can occur because of data mismatches between operator systems, technical faults in the porting database, incorrect account details submitted by the customer or gaining operator, or internal processing errors at either operator. Whatever the cause, the effect is the same: the customer’s number remains on the old network after the agreed date, leaving them unable to use the new SIM as planned.
If a port does not complete on time, the first step is to contact the gaining operator immediately; as the party who initiated the request, it bears primary responsibility for the process. Give the operator your PAC code, the agreed port date, and your account details. Most genuine technical delays can be resolved within hours once escalated internally. If the gaining operator blames the losing operator, ask for that to be confirmed in writing and follow up with the losing operator as well. Document every contact.
| Problem | Common cause | Responsible party | Immediate fix |
|---|---|---|---|
| Port not completed on time | Data mismatch; system error | Gaining operator (primary) | Contact gaining operator; escalate to complaints |
| Both SIMs active simultaneously | Transition overlap in porting database | Both operators; gaining operator to resolve | Keep old SIM in safe place; do not cancel manually |
| Number lost / deactivated | Processing error; number recycled prematurely | Losing or gaining operator depending on cause | Escalate urgently; request port reversal |
| Port refused without valid reason | Losing operator obstruction; outstanding debt dispute | Losing operator | Clarify outstanding balance; formal complaint if none |
| PAC code not delivered within 60 seconds | Network error; account query on losing side | Losing operator | Retry; contact losing operator if persistent |
Both SIMs active at the same time
A brief period during which both the old and new SIMs register on their respective networks is technically expected during the handover window; calls and messages may reach one or the other. In most cases this transition resolves itself within a few hours as the number is definitively re-anchored in the porting database. However, if both SIMs remain active for more than a working day, or if calls and texts are inconsistently delivered across the two numbers, this constitutes a porting failure.
Do not attempt to manually cancel the old SIM yourself during this period, as doing so may complicate the technical resolution. Instead, contact the gaining operator and report the situation. Keep the old SIM somewhere safe in case the port needs to be reversed. Note which SIM receives which calls or messages and at what times, as this information can be useful for the operator’s technical team and for any subsequent formal complaint.
When a number is lost during porting
In rare cases a porting error results in a number being deactivated rather than transferred — meaning it no longer exists on any network. This is the most serious porting failure. It can arise from a number being incorrectly marked as available for recycling in the industry databases, a processing error at either operator, or a combination of account-level mistakes. Once a number is released for recycling it may be reassigned to a new customer, which complicates recovery.
If you suspect your number has been lost, escalate to both operators urgently and use the word “formal complaint” in writing immediately. Operators are required by Ofcom to take reasonable steps to restore a number where a porting error is the cause. Request a port reversal in writing. If the operators cannot restore the number, or if there is a dispute about responsibility, the matter should go to the relevant ADR scheme — CISAS or the Communications Ombudsman — as quickly as possible, given the risk that any delay reduces the chance of recovery.
Operator responsibilities and Ofcom rules
Ofcom’s General Conditions place clear obligations on both the losing and gaining operator in any port. The gaining operator is primarily responsible for initiating and completing the port correctly. The losing operator must not obstruct a port other than in limited circumstances — principally where there is a genuine outstanding debt (though even then the process for handling this is regulated). An operator cannot delay or refuse a port simply to retain a customer or as a commercial strategy; doing so is a breach of the General Conditions and may be referred to Ofcom.
Ofcom monitors operator compliance with porting rules through its annual Connected Nations and complaints data reports. Customers who believe a port failure reflects systemic non-compliance rather than an isolated error can raise a concern directly with Ofcom, though Ofcom does not adjudicate individual disputes — that function belongs to the ADR schemes.
How to escalate a porting failure
If the gaining operator cannot resolve a porting failure promptly, escalate to a formal complaint with that operator. Follow the standard mobile complaint process: lodge a formal written complaint, obtain a complaint reference number, and allow the operator up to eight weeks to resolve the matter. If the complaint is unresolved at the eight-week point, or if you receive a deadlock letter sooner, you may refer the dispute to the relevant ADR scheme free of charge.
In parallel, you may also raise a concern with Ofcom using its online complaints tool. Ofcom uses complaint data to monitor operator compliance and may investigate if a pattern emerges, though individual dispute resolution remains with CISAS or the Communications Ombudsman. Where a porting failure has caused demonstrable financial loss — for example, missed business calls, additional costs for a temporary SIM, or charges on both accounts simultaneously — document this carefully as it may support a compensation claim through ADR.
What this means in practice
James lives in Leeds and switches mobile operator on 3 March 2026. He texts “PAC” to 65075 and receives a code within a minute. He provides it to his new operator with a requested port date of 10 March. On the morning of 10 March, his new SIM shows no signal and his old SIM still registers on the old network. James contacts his new operator by email at 09:30, using the word “complaint”, and requests the port be completed urgently. The operator’s technical team confirms a data mismatch on the account and resolves the port by 14:00 that same day. James notes the complaint reference number. Because the matter was resolved within the same business day, he does not need to escalate further, but he keeps the correspondence in case a billing dispute arises regarding double-charging during the overlap.
Related Guides
How we verified this
This article draws on Ofcom’s General Conditions of Entitlement covering number portability, Ofcom’s 2019 switching reforms and associated guidance, Ofcom’s Connected Nations reports, and the published rules of the CISAS and Communications Ombudsman ADR schemes. No operator-specific claims have been made.
Disclaimer: Kaeltripton.com is an independent UK editorial publisher. We are not regulated by Ofcom or the FCA and we do not sell or arrange mobile services, insurance, or financial products. This content is for general information only and is not legal, financial, or technical advice. Rules, prices, and operator policies change. Verify the current position with Ofcom, GOV.UK, the ICO, or your provider before acting. ICO registered ZC135439. Last reviewed: 2026-06-05.
Frequently Asked Questions
What if my number does not port on time?
Contact the gaining operator (the one you are moving to) immediately using the word “complaint” in writing, and provide your PAC code and agreed port date. The gaining operator is primarily responsible for completing the port and should escalate internally. Most delays are resolved within hours once flagged. Keep a record of all contacts; if the matter is not resolved within a reasonable timeframe, escalate to a formal complaint and then to ADR if necessary.
What if I have two active SIMs after porting?
A brief overlap during the technical handover is expected and normally resolves itself within hours. If both SIMs remain active for more than one business day, or calls and messages are inconsistently routed, contact the gaining operator and report it as a porting failure. Do not cancel the old SIM yourself during this period, as this may interfere with the technical resolution. Keep a note of which SIM receives which communications and at what times.
What if my mobile number is lost during porting?
Escalate urgently to both operators in writing, using the word “formal complaint”. Request a port reversal immediately. Operators are required by Ofcom to take reasonable steps to restore a number lost due to a porting error. Time matters: if the number enters a recycling pool it may be reassigned. If operators cannot resolve the matter promptly, refer to the relevant ADR scheme — CISAS or the Communications Ombudsman — without delay.
Who is responsible if a number port fails?
The gaining operator — the one you are switching to — bears primary responsibility for initiating and completing a successful port. However, the losing operator has obligations too: it must not obstruct a port and must cooperate with technical processes. Responsibility for a specific failure may lie with one or both parties depending on the root cause. The gaining operator is generally your first point of contact and escalation.
How do I complain about a failed number port?
Lodge a formal written complaint with the gaining operator, using the word “complaint” and including your complaint reference, PAC code, agreed port date, and a summary of what went wrong. The operator has up to eight weeks to resolve the matter. If it issues a deadlock letter sooner, or eight weeks pass without resolution, you may refer the dispute free of charge to CISAS or the Communications Ombudsman, depending on which scheme your operator is a member of.
