We had this issue when we first installed our system, but only happened on 1 of our POTS lines.
Turned out to be a bad connection from the line to the gateway. We re-terminated it and it worked fine. It was very strange because once in a while it worked, but most times if a call came in on that line it would ring and you would answer, but no one was there.
Re-punching the line took care of it, it never happened again.