Call not getting Registered

Hi,
We have setup asterisk along with a2billing and its working fine, except for 1 major issue.
If a call is made, user answers the call, speaks for a minute or two and disconnects, the call time is registered and equivalent amount is deducted from his account.
BUT,
if the call is disconnected from the SIP client end, the call time shown is 00:00. And no amount is deducted.
We tried with various other SIP clients, but the issue still persists.
Need a slight push in the right direction.

Regards,

  • Mj