Settings > Call Signalling > Ring Signal

Ring Signal
If the ring signal is enabled, calls are acoustically signalled by a ring signal. If you don't want your environment to be disturbed by the ringer on the telephone, you can switch off the ring signal.
This function is different than the do-not-disturb function on the PBX in that calls are still possible. However, they are no longer signalled acoustically but rather visually.
Proceed as follows:
Using COMfortel Set (page Settings/functions > Signalling/ringer, check box Ring signal)
Using the telephone (menu functions > call signal/light)

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