There are 2 main ways that postpaid mobile bills are handled:
Manual. A bill will be sent to you at the end of each month that includes your plan fee and any other charges. You can usually pay by a variety of methods.
Automatic. Your bill will be automatically paid by direct debit or credit/debit card at the start or end of each month. Additional fees may be charged mid-month, or added to your end-of-month bill