You can only mute the handset ringer if the handset is off the cradle when the phone starts ringing.
Muting the Ringer
Do Not Disturb (DND)
The do not disturb feature (DND) allows you to mute the ringer of the base and any registered handsets at the same time. The phone must be in standby mode. Press and hold do not disturb on the base. You will hear a confirmation tone, and the DND LED illuminates. To cancel the DND feature, press do not disturb again. You can also mute the ringer tone while the phone is ringing by pressing do not disturb on the base.
Temporarily Muting the Ringer
To mute the ringer tone temporarily for each handset or the base, while the phone is ringing, press end on the handset or the MUTE soft key on the base. The mute will last for the current incoming call only. The ringer tone will return to the previous setting starting with the next incoming call.
Audio Tones (Base Only)
The audio tone feature provides three different tone levels. If you have difficulty hearing a caller:
From the 
/volume up/down to select Audio Tone Setup, and press the OK soft key. Each press of the CHANGE soft key will change the audio tone level. The default setting, Natural Tone, is recommended for hearing aid users.
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