More important than the button is the status indicator - I need to know if the call is muted or not. Even better, promote it to an OS-level icon/badge/overlay. If my mic is actively in use, please make it blindingly obvious.
And then I have to keep hovering over the icon and guessing from the tooltip whether I'm muted or not. Unfortunately, different software tends to be inconsistent with toggle buttons - sometimes the icon tells you what is, sometimes it tells you what will happen if you click it.