

It’s a legal requirement in the UK. Mobile number is used in so many places it’s effectively acting as an ID.
This fosters consumer choice and supports effective competition by removing barriers to switching – being able to keep your number may impact your decision to change provider.
https://www.ofcom.org.uk/phones-and-broadband/phone-numbers/number-portability-info

Thanks for the reply, I will checkout Wallabag.