Hitting Play/Pause key (F8) ends FaceTime call
You mentioned in the comment that the music playing app is usually Spotify.

The media keys (Previous, Play/Pause, Next represented by F7, F8 and F9 keys respectively) gets associated with the an app outputting audio/video (if the app is programmed to support it).

If two or more apps are simultaneously outputting audio/video, the keypress is captured by Apple's own built in app (in this case FaceTime).

This explains your observation. In this case, it would be recommended to pause Spotify using it's menu command.