Settings > Receiver and Headset > Receiver Volume

Receiver Volume
Whenever necessary, the microphone volume of the receiver can be increased or decreased.
Note: Press the right or left navigation key to configure the desired microphone volume.
Proceed as follows:
Using the telephone (menu settings > def. settings > receiver volume)

COMfortel 1400 - Firmware V2.2 - COMfortel Set V3.16 - Advanced Information V07 02/2019