i can see 3 main possibilities :
– radio interferences : check if other cells around are using same frequencies
– handover : during a handover the voice is cut, as long as the MS is trying to connect to the target cell. It takes usually less than 100ms, but in case of interferences, it can take longer time !
– i would also check the transcoder alarms
have you been able to reproduce the problem with a drive test tool ? if yes, then you can check the rxqual, FER, and what’s going on during the handover.
Ater congestion cannot be the cause (a circuit is either allocated or not, but it doesn’t de-allocated during a call : it’s a static allocation at call setup)