A2Billing - Fake ring

As soon as a call is passed, it is possible for A2Billing to tell Asterisk to pass back a ringing tone to the caller, while the call is being processed.
Asterisk - Dial command - Option "r"
r: Generate a ringing tone for the calling party, passing no audio from the called channel(s) until one answers. Without this option, Asterisk will generate ring tones automatically where it is appropriate to do so; however, "r" will force Asterisk to generate ring tones, even if it is not appropriate.
Disable this option
In the agi-conf that you are using for the calls, remove the "r" option.