why the call terminated after the call connected?

I have a problem,I make a call between my application and the gateway which 
type is Cisco 5300,but the call suddenly terminated when the call connected 
a while.I also capture the packet,The call accomplished TCS and MSD.Both 
side reply TCS ACK and MSD ACK,But after a while,Cisco gateway send a 
message TCSRelease,then the call terminated.TCS have completed well, why 
does cap timer expiry?
(When I use no fast and no h245tunnel,the call has no problem. )

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