Wait, are you seriously using Roll20's voice chat? God help you. In any case, I can't imagine any way to do this that isn't incredibly cumbersome and frustrating. It's not a feature I've ever seen in any voice chat program, and Roll20 isn't a voice chat program, it's just the most basic possible mic input and nothing else. It doesn't even have settings for push-to-talk or noise cancellation, or an indicator for whether your mic is working or not, and you want them to invent features that Discord hasn't figured out how to do?