Thanks for the quick response, Pent.
It turns out it was a bit of user error and partial functionality with
the phone app.
If mute is set via facing the phone face down prior to an incoming
call the mute button is highlighted and visually displays mute. While
a call is coming in, moving the phone face up DOES toggle mute but the
button doesn't update to reflect this.
If the call is already in progress the button doesn't visually update
when the Mic is toggled, but the mute Mic function is definitely
working.
It would be extremely useful to have the button update appropriately
to see a standing status of the mute function. I expect that is the
phone app and not tasker?