Where is the "MUTE" button on the T22P?

[Abigail]

Good day.

Occasionally, I need to turn off the microphone on the phone and the hand set, but haven't figured out how to do it.

The T22P User Manual v. 60.0 has many references to a "MUTE" button.

I have closely examined my phone and cannot find a hard key for the same (although there is a microphone icon below the "X" button).

I also cannot see a soft key come up while dialing or after the call is established.

There is a very unclear discussion of "Action URI" on page 44 of the manual that also mentions the "MUTE Key".

A search of this forum brings up nothing specifically on point

Has anyone found the "MUTE" button or knows how to make the "MUTE" key appear?

Thank you.