When a UK mobile number is ported, the number is reassigned to the new provider's SIM or eSIM. The old physical card does not move. Once the port and old-service closure are complete, the old SIM should no longer carry that number.
During the porting window
There may be a short period when calls, texts and data behave inconsistently between the old and new lines. Keep both phones or SIMs available, avoid making account-security changes during the switch, and do not destroy the old card until the new line has been fully tested.
How to confirm completion
- Restart the new phone when the provider says the port is complete.
- Make and receive an ordinary call using the ported number.
- Send and receive an SMS.
- Turn Wi-Fi off and test mobile data.
- Check voicemail and any services tied to the number.
- Confirm the old SIM no longer receives normal service.
Does porting close the old account?
Using a PAC normally triggers the mobile switch, but you should still check the final bill, any linked device finance, add-ons, multi-line services, outstanding credit and Direct Debit. Do not assume every separate agreement attached to the old provider ends automatically.
PAYG credit
Remaining PAYG credit does not normally move with the telephone number. Check the old provider's current refund or forfeiture terms before porting.
Two-factor authentication
SMS codes should begin arriving on the new SIM after the port, but temporary delays can occur. Before switching, save recovery codes and make sure important accounts have another recovery route.
What to do with the old physical SIM
After the port is verified and any account issue is resolved, cut through the chip and dispose of the pieces securely. Do not sell, return or give away a phone with the old SIM still inside unless the provider specifically requires its return.
If the old SIM still works
Do not assume this means you have two active copies of the same number. The port may still be processing, or the old account may have another line. Contact the providers if both lines continue to behave unexpectedly after the stated completion time.
How to tell whether the change actually worked
Keep the old SIM until the new line passes incoming and outgoing call, SMS, data and voicemail checks. The old card normally stops carrying the number after a completed port, but the old account may still have a final bill, PAYG balance or separate device finance. Destroy the card only after those records are settled.
- Check the outgoing number on the new SIM.
- Receive a test SMS from another network.
- Do not cancel the old Direct Debit before the final statement is confirmed.
Official information checked
Information checked on 25 July 2026. Porting, final billing and PAYG-credit treatment depend on the providers and account agreements.
