AASTRA 4225 for MX-ONE Telephony Switch - Manual (Page 108)

Settings
Dialog 4223 Professional/Dialog 4225 Vision 108
Ring signal volume
Use the Volume keys to adjust the ring signal volume when the phone is
in idle mode or ringing. Adjusted volume is stored.
ì
Press to adjust the volume.
Mute ring signal
You can suppress the ring signal for an incoming call.
É
Press to suppress the ring signal.
The ring signal is switched off for the current call, and your phone is
automatically set to Silent Ringing (see below).
Silent Ringing
If you do not want to be disturbed by the ring signal but still want to
answer an incoming call, you can switch off the ring signal. Incoming
calls are only indicated by a flashing line lamp and display information.
When the phone is in idle mode, or when ringing:
É
Press to switch off the ring signal.
The Mute key lamp and the Ringer off symbol are switched on to
indicate Silent Ringing. The ring signal will automatically be switched
on the next time you Lift the handset or press any key.