Call disconnection over GSM network may happen due to radio link timeout, or handover failure, etc. What are you interested, for example the condition for call disconnection, or the process between MS and the network? Pls desribe your question in more specific scenario so that I can help you. You can refer to GSM 04.08 (RR connnection release procedure), 05.08 (radio subsystem link control), 03.09 (handover) for more information.
Nguyen