It's actually a terrible system though. It relies on the original issuer of the number maintaining essentially a "proxy" to your new provider. So if I transferred from o2 -> EE, and o2 had network issues, it's possible (and has happened to me personally) that I am affected even though I have left o2. I think also calls are actually physically routed through the original network. Lots of weird things can happen like texts from certain networks never arriving.

I had to get a new number when I discovered my NZ family could receive my texts but their replies never arrived at my end.

For more info, see https://www.theregister.co.uk/2017/07/20/uk_number_porting_c...