Post-paid mobile service is when you are billed for actual usage and overages AFTER they are incurred. Monthly recurring fees (the amount of your regular monthly fee per line) is billed a month in advance prior to usage.
For example, you start service on 1/1. Your invoice will show billing for 1/1 – 2/1 for your monthly recurring service that will be due by 1/28, however if you make international calls on 1/13 and 1/14, these charges will not display until your 2/1 bill which will showing these international charges and your monthly recurring fee for 2/1 – 3/1.