There's no account number with 48.
Your account is identified by your phone number/email address.
Are you looking for it because you're trying to port your number from 48 to another provider?
If so then if it's asking for an account number then you're choosing the wrong option.
You need to choose prepaid registered rather than billpay etc.
If you need to contact 48 support about this then see the links in my signature below.
