Logs show that the Shoretel is sending a "BYE" when the call is answered... I am on 12.3 currently, will be moving to 14.2 once (if ever) it's stable.
I probably can't offer too much help, but I would like to mention that Shoretel made a lot of improvements to SIP trunks starting with version 13.1.
Ver 14.2 had some pretty rough builds with some major issues for a couple of customers. It has been out for a while, so most of the major problems should be fixed by now.
I know that an upgrade of that size is a pretty big task, and I can't promise that the upgrade will fix your problem.
