Sometimes it’s difficult to determine why a call is failing in A2Billing. It’s possible to turn on some very detailed logging from the A2Billing script that can often be used to track down the issue.
The option to turn on debug logging (in A2Billing v1.4+) can be found under System Settings / Global List / agi-confX. You need to turn it on for the agi-conf that your call will be using. Turn logging up to 4 like this –