While charging the handset you cannot mute the ringer tone for the handset.
Adjusting the Ringer, Earpiece and Speaker Volume
Ringer volume
In standby mode press 
/vol/ 
or 
/vol/ 
on the handset to select one of three ringer volumes (Off, Low or High)
Earpiece and Speaker Volume
You can select earpiece volume from among four volume levels on the handset earpiece and six levels on the handset
speaker. Pressing the volume up key or volume down key on the handset during a call will change the earpiece or speaker
volume. This setting will remain in effect after the telephone call has ended.
If you press the volume up key when the earpiece is at the maximum volume level, an error tone sounds. The error tone also sounds if you press the volume down key at the lowest volume.
Temporarily Muting the Ringer
To mute the ringer tone temporarily for each handset, while the phone
is ringing, press the MUTE soft key on the handset you want to mute. 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.
[31]