What exactly do you mean?
If you want to port a number from another provider to 48 then follow the instructions here:
If you want a different 089 number from 48 for your existing My48 account (why?) then you'll probably have to contact 48 support to see if they can do this - although I've never heard of them changing the number on an existing user's 48 account.
Otherwise an obvious way to get a new number is to get a brand new SIM, register/activate it using a different email address to your existing My48 account and then you'll get a new 089 number.
If you need to contact 48 support about this then see the links in my signature below.
