Add grace seconds to specific trunk

I have deployed Astbill successfully using multiple trunks. We also have a specific trunk which is just a simple FXO SIP device (HT503). This device does not really tell Asterisk or Astbill when the call at the far end really got connected. It does do busy detect and hang up correctly. This means there could be inaccuracies in billing. This inaccuracy only exists for this specific FXO trunk and not others. So my question is, is there a way to fix this by providing "grace" or included seconds only for this specific trunk.

Your input is appreciated.

regards,

ujar62