I'm not sure if this will help anybody or if it has been posted before but here goes:- :)
Every year I get a new Mobile contract. For the last four years it has been with 3 Network. You cannot 'port' an existing '3' number back to '3' so I get a PAC (Porting authorization Code) from 3 and transfer that to a PAYG O2 Sim. After two days I get the PAC from O2 and port it to 3 network.
A bit of messing about you may think but I get a new and better 3 contract every year and I keep the same number I have had for five years.
I'm not sure if this will work with other companies but I don't see why not.
Sorry for the long post :oops: