Below are links to the 3 parts of a post covering integrating OpenSIPS with Asterisk and A2Billing. The setup described uses OpenSIPS handling A2Billing customer SIP requests for calls (for example when providing a wholesale service to other SIP PBXs).
It’s based on OpenSIPS v1.8 / Asterisk v1.8 / A2Billing v2.01
Please take security in to consideration with VOIP whether testing or in a live situation.
Part 3 – The opensips.cfg config file