Apple says their messages and FaceTime calls are end-to-end encrypted and nobody can access them.
As I'm unfamiliar with cryptology, I've found security white paper from Apple, where says:
FaceTime uses Internet Connectivity Establishment (ICE) to establish a peer-to-peer connection between devices. Using Session Initiation Protocol (SIP) messages, the devices verify their identity certificates and establish a shared secret for each session. The cryptographic nonces supplied by each device are combined to salt keys for each of the media channels, which are streamed via Secure Real Time Protocol (SRTP) using AES-256 encryption.
How secure is FaceTime and is it really impossible to decrypt/access/listen calls and video?