you can create a specific agi-confx set in a2billing.conf and in that agi-confx, simply modify the dialcommand_param option and make it look like this:
Code:
dialcommand_param = "|60|HRgrL(240000:61000:00000)"
This will limit all calls to 4 minutes (240,000 milliseconds) and there will be an announcement 60 seconds before the call is disconnected.
Than use that agi-confx in the asterisk DeadAGI() when calling a2billing. You can also not create any new agi-confx and make this the default behavior of you a2billing calls all together.