Apple says their messages and FaceTime calls are end-to-end encrypted and nobody can access them.

Are FaceTime calls secure?

“FaceTime is Apple's video and audio calling service. ... The audio/video contents of FaceTime calls are protected by end-to-end encryption, so no one but the sender and receiver can access them. Apple cannot decrypt the data.

Can FaceTime calls be monitored?

FaceTime is private because your calls are protected using end-to-end encryption, so there is no way someone outside of your call (potential hackers) could access your call. Calls are not recorded, and no part of your calls are sent to or stored by Apple. Only you and the person you call can join the call.