There is a thread talking about muting the mic by default so you wouldn't have to select this every time: https://answers.microsoft.com/en-us/msteams/forum/all/teams-mute-by-default-when-entering-a-meeting/bf6613f2-f105-44e9-b448-db7c106272ec
It has a link to this idea so anyone can upvote it. Unfortunately, it looks like the idea has been deleted. When you click the link, it takes you to a 404 page. Is there another idea page for this?