Port number

Order a free Asda sim online, order your new contract which will give you a new number.

Port current number to Asda, then when that’s completed port back onto the new contract.

I’ve found Asda customer service to be top notch, always get through to a human without loss of menus if you get stuck or need more info.

Take a pay as you go sim out on the cheap, port away then port back after.