Assuning these are VoIP customers, You could also create an extension in the a2billing context that called a different agi-conf that read out the balance when you dialed 123.
Code:
[a2billing]
exten => _x.,1,deadagi(a2billing.php|1)
exten => _x.,2,hangup
exten => 123,1,deadagi(a2billing.php|2)
exten => 123,2,hangup
Alternatively, if these were call through customers, and you are using 1.4, you could create a DID with a destination of local/balance@read-balance, and the code in the asterisk dialplan would be something like -
Code:
[read-balance]
exten => balance,1,deadagi(a2billing.php|2)
exten => balance,2,hangup
Joe