We are asterisk/a2billing/voip solution provider and have setup various system for calling, sms, conference, calling card, pbx, fax and many more services.
We have been using a2billing for various projects and are aware of all its capability and functionality. Currently it has admin, customer and agent panels and the reseller functionality can be easily managed via admin panel. We can give option in admin to create reseller and assign rate card to each of them. In reseller panel, reseller can further add his margin on the rate card and can generate pins. There customers will be charged as per their rate card.