Settings > Receiver and Headset > Headset Operation On/Off

Headset Operation On/Off
If you have connected a headset to your system telephone and selected it, you can switch between receiver and headset operation.
The telephone detects the connection of a headset and, in this case, the telephone switches the headset operation on. The telephone detects the disconnection of a headset and, in this case, the telephone switches the headset operation off.
 
Proceed as follows:

COMfortel 1600 - Firmware V2.2 - COMfortel Set V3.16 - Advanced Information V10 02/2019