Although the microphone icon indicated "muted", others can still hear me.
1) Join Teams Meeting
2) Mute microphone
3) "hey, can anyone still hear me?"
4) Other users acknowledge they can still hear
5) Verify the microphone icon has a strike through it, and there is a message above indicating microphone is muted
After reading a similar issue in a forum I tried disabling and re-enabling the microphone via settings for the Teams app. This has (for now) solved the problem. However, this is a serious problem. The app must be better at confirming that the microphone is actually muted.