Additional meeting and call-related features and issues within Microsoft Teams for business
Hi! This is Nicolle an Independent advisor.
Usually, before you join a meeting in teams there should be a window that will appear on your screen for an option if you are mute or unmute before you click the "join" button.
You can find also find useful answers in regards to that:
In other words, you have to remember every time you join a meeting - this needs to change, to elevate MS to Zoom!