Handset key nomenclature

Question

the user guide that was packed with my NeoBio 10/15r/20/25r tells me that it has a mute button C but neither the guide itself nor the handset, identifies one. This causes simpletons like me a problem but I don't want to believe that I am unique and I have already seen more settings that seem to need keys I can't find. Is there a simple solution to my problem? any advice, even fatuous would at least give me a clue as to what I am up against. Many thanks in anticipation. Kind regards. Geoff t