Settings > Sound > Settings ringtone

Settings ringtone
Proceed as follows:
Using the telephone (page  > Functions > Ringtone)
 Calls will be signalled acoustically by a ringtone. This function differs from the PBX function do-not-disturb insofar that calls are still admitted but they will not be signalled acoustically but visually.

COMfortel 1400 IP/2600 IP/3600 IP - Firmware V2.8 - Advanced Information V08 12/2020