On the FON-175, the MUTE button does more than just mute the microphone, as I recently discovered. The MUTE button also places the phone into "Silent Ring Mode."
When the MUTE button is activated while NOT in a call (that is, when the handset is on the base), the MUTE button will toggle the Silent Ring Mode feature. This prevents the phone from ringing audibly during a call. The red light will flash during an incoming call, but the phone will not ring.
In order to toggle the setting back again, press the MUTE button while the handset is in the base and is not in use.
Hope this helps, as this terminology (Silent Ring Mode) is NOT referenced in the user's guide.