TDM400P Hangup Issues

im having difficulty with my TDM400 on asterisk 1.6. When i place an inbound (both mobile & pstn)call to a SIP phone or vice versa, everything works fine.Therefore my gsm gateway or pstn trunk is released immediately after i hangup. However when a inbound call is not answered and the call is diverted to another PSTN or mobile number the trunks are not released. I can only achieve a release of the lines by manually disconnecting the trunk lines say to the pstn or to my gsm gateway…this is driving driving me nuts…can someone out there suggest a solution for me.