[Asterisk-Users] Phone ringing and not going to voicemail?

[Matt]

Hi,
I have one phone on my network that just keeps ringing (when I call
it) and does not go to voicemail.

If the person there is on the phone, and someone calls it they get the
busy message, but they never seem to get the 'unavailable' message...
instead it will just ring and ring and ring... any ideas?

They are setup with a voicemailbox, and it is set to transfer after 15
seconds of ringing.