I use a2billing (underlying asterisk) for calling card and VoIP (sip clients) services. I need a functionality that when a sip client tries to dial a number from his sip phone and when the balance is low or there is no balance (to be decided) php informs the customer via voice prompt (ivr) that the balance is low ( or no balance) so the customer can expect that either will not be able to make a call or the call might be cut off soon due to the low balance. This low balance value should be changeable according to our needs. This feature should not interfere with calling card only customers.
We would need to be in touch to discuss more details and I need to see where the changes are made for future support.