Settings > Receiver and Headset > Headset Volume

Headset Volume
Whenever necessary, the headset volume can be used to adjust the microphone amplifier on the connected headset upwards (if your communication partner cannot understand you) or downwards (if speech is distorted).
Proceed as follows:
Using the telephone (menu settings > headset > headset volume)
Note: Press the right or left navigation key to configure the desired microphone amplifier.

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