[Serusers] BYE method not handled properly

I would like to know why does my BYE method are always replied with a 'Call Leg/Transaction does not exist' . How do they compare whether the transaction in the BYE method exist or not ? ( tag? ftag ? ) Are there any thing in the config that might cause this kind of problem ? Just want to highlight that all the calls are made in a good condition, everything except when the call is ending. I have checked that the phones have not received a prior BYE. Any other idea what is wrong ? Here's a more detailed situation :- Caller (PSTN) --> Voice Gateway --> OPENSER --> Callee (UA) When Callee (UA) tried to end the call , OPENSER will forward a copy of the BYE to Voice Gateway to inform him of the BYE. The Gateway , somehow , replied with a 'Call Leg/Transaction Does Not Exist' . The strange thing is, the Caller (PSTN) was somehow informed of the BYE method and terminate the session . Anyone has any idea how to handle these errors ? I will be glad to provide a ngrep for more reference. Please let me know if you dont understand. Regards, Sam