A2Billing customization

We are a VoIP company in the US looking for a PHP programmer to customize the open source A2Billing (based on Asterisk)project to allow for the billing of our origination (incoming DID) minutes. System already is setup and functions for calls made out by our customers, but we basically need to replicate the same scripts and processes for incoming calls. Baiscally, duplicate the billing engine for inbound calls as A2billing does not currently rate VoIP calls, and we need to emulate a PSTN trunk while using our VoIP trunks to accomplish this. Will need the steps documented and provided to us upon completion.