AASTRA 4420 IP Basic (SIP) for MX-ONE User Guide - Manual (Page 54)

Settings
Dialog 4420 IP Basic 54
Silent Ringing
If you do not want to be disturbed by the ring signal but still want to
be able to answer an incoming call, you can switch off the ring
signal. Incoming calls are only indicated by a flashing line lamp.
When the phone is in idle mode, or when ringing: É
Press to switch off the ring signal.
The Mute key lamp is 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.