Hi Phillip,
If the line is disabled it will not register. If the line is set to outbound it will register too. There does seem to be an issue when the line is set as an inbound VoIP line.
Thanks for pointing this one out, at the moment there is no remedy exept to leave it as an in&out line. Just do not assign a 'outbound rule' for it. this should be rectified by the next build.