Check if MCU sends ATH command to close the call.
Check whether there are capacitors with high capacitance like more than 1uF in the audio interface. In general，only 10pf and 33pf capacitor can be connected to the audio interface to eliminate the RF interference. Capacitor with high capacitance will cause the instability of audio PA, which may lead to the call hanged off.
Depending on module used - if another event uses GPRS during voice call, the call may be disconnected.
All the best: