So I imagine you are using OpenSer to both solve NAT problems with MediaProxy, and to load balance between the the A2Billing servers.
Yes, I am using mediaproxy for user behind nat and that works very well. About load balance, i am not sure what is the right way to do. what i am doing now is call to usa/canada will go to a2billng 1, call to hong kong/china will go to a2billing 2. I set this on the LCR table in openser.
1. Unified login - Sign up to A2Billing but authenticate your SIP endpoint to OpenSER. Looking at the openser database, you may have to add some fields to Sip_buddies - but I do not think that should affect the overall operation of A2Billing - but testing will prove this.
2. Authentication from OpenSER to A2Billing, but I suppose you could use the equivelant of CLI recognition for that, unless you have found something better?
This 2 points are what i am working on. I am thinking to have user to pick their own usename and password for openser, and openser will have a unique callerid when the call being sent to a2billing, then in the cc_callid table it map the callerid with the card account.
Still trying to work through this. I will post all my conf once i get it working.