NEC SV9500 Ringer Mute feature

Maybe you tried it too fast. Potentially you have to re-register the phone (ie unplug and plug it back in). Otherwise, the manual literally only has this one command that you quoted for it, so there shouldn't be anything else. I use it too and it works on my system.

Thank you for your answer. I did that (unplugged it and plugged it back) and that didn't do anything. I found that the phone I was testing with was assigned as 24 Line in ADKS and I changed that to 16 Line but that didn't help either. Don't know if it has something to do that it is 6 button phone but the feature is there and you can press it and activate it, just nothing happens.
I can try set up 24 or 32 button phone and see if it works on those? Not sure what else to check?