Part 4 of the quick start video guide for A2Billing shows how to create a SIP enabled customer. This could be used for residential where you are billing a customer connecting their handset directly to your system, or maybe a customer who is connecting another PBX to your system using SIP.
Please not that if you are enabling SIP calling you should check out this security post and make the relevant change – http://sysadminman.net/blog/2011/a2billing-blind-transfer-and-fraud-2823
We also have a quick look at some of the agi-conf settings that affect how this call is handled.
The system used was a clean install of a SysAdminMan virtual server. You can find more details about this here – http://sysadminman.net/sysadminman-freepbx-a2billing-hosting.html