1. Check if the MCU has sent an ATH command to end the call
2. Check whether there are capacitors with high capacitance (i.e. more than 1uF) in the audio interface. In general, only 10pf and 33pf capacitors can be connected to the audio interface to eliminate RF interference. Capacitors with high capacitance will cause instability to the audio PA, which may lead to a call hanging up