There's no account number with 48.
Your account is identified by your phone number and email address.
If you're trying to port your number from 48 to another provider then you're probably choosing the wrong option.
You need to select prepay registered and just enter your phone number.
If you're trying to do something else then you'll need to explain more clearly what it is.
If you need to contact 48 support about this then see the links in my signature below.
