Accept a video or audio call and end a current call: Click End & Accept.
Accept an audio call and put the current audio call on hold: Click Hold & Accept. When you end the new call, the call on hold resumes. You can pause one audio call at a time.
Join a Group FaceTime call: Click Join, then click the Join Video button signed in to iMessage.
Decline a call and set a reminder to call back later: Click Return recent or missed FaceTime calls on MacApple Support article: If FaceTime isn’t working on your Mac