Unable to get inbound calls automatically/manually forwarded to voicemail.

[[email protected].]

Having set up the solution for one user/extension, when inbound calls are received, it doesn't automatically go to voicemail if not answered or the 'voicemail' button pressed on the receiver device. Call goes silent for a moment and then rings again before cutting out. Has anyone else encountered this issue, and if so, how did you resolve it?