Why is an app using my microphone when I make a call ?

It’s the phone app… if it didn’t use the microphone, the party on the other end would not be able to hear you.

The orange dot was added when iOS 14 was released. It should appear on any iPhone running iOS 14 or later when the Phone app (or any other app using the microphone) is in use. See: About the orange and green indicators in your iPhone status bar - Apple Support

When you make a call, the Phone app uses the microphone.