The only way to use it is to buy memberships or add-ons. If you're getting a new phone why not just get a replacement SIM that's linked to your existing account and that way you keep your credit. If you want/need a new number 48 support can probably arrange that for you if you ask them.
Hope this helps.
I am not a 48 employee so if you need to contact 48 support then use option 3 or the chat facility here.