You can't transfer one pay monthly number from one subscription to take over another.
We'd recommend changing the ownership of the number and subscription from your mum's name into yours. But, it sounds like it would mean you have two subscriptions; the one you've newly taken out in your name and the one that you'd transfer from your Mum's account.
Our Mobile Care team would be able to take a look at the way it's set up and advise you best going forward.
It should be possible to get a PAC for your old number (if it's out of the minimum term), and PAC out to another network PAYG SIM, then PAC in to replace your number?
