on 21-10-2020 14:57
21-10-2020 15:05 - edited 21-10-2020 15:06
21-10-2020 15:05 - edited 21-10-2020 15:06
Assuming O2 to O2 you need to do a sim swap Guide: Sim Swap: a mini guide 2017 update
If coming from another network you need to get your PAC Guide: How to get your PAC [June 2021 Update] and complete the request form here https://www.o2.co.uk/help/phones-sims-and-devices/how-to-keep-your-mobile-number