This app was mentioned in 2 comments, with an average of 1.00 upvote
If you are talking about while you're on the phone then I'd recommend this app https://play.google.com/store/apps/details?id=com.curlyygroup.callbuoys It floats buttons over your screen allowing you to do other things while having quick toggles such as a mute, speaker, hide, and a hang up button.
~~Blimps~~
My mistake, I thought one was a mute button.
Second Edit: Also, it looks like you may be looking at a Root/Xposed app to accomplish this after some reading into it.