FaceTime doesn't complete connection

Abour 2 days ago, FaceTIme stopped making connections. It launches, dials the recipient but then doesn't complete the connection. We have two iMacs and they won't connect to each other (it rings but FaceTIme doesn't launch) nor connect to someone else who we've contacted daily. Both iMacs are running OSX 10.11.6. All other internet connections are working fine. Any suggestions?

Thanks