Nothing gets sent to the siperator and as a result the siperator sends a 408 request timeout to the service provider, dropping the link. The shoregear switch then responds to the IP set with a 507 status : 409 server internal error - re-invite inside a re-invite.
Packet captures indicate a 15 minute keep alive reinvite being sent to the siperator, to the shoregear switch, to the IP set then the IP set responds with an invite of it's own. Have not tested calls directly answered by a 480G set. In calls answered by the auto attendant and routed to their destination do not drop at the 15 minute mark. Pick up a polyccom conference phone (registered to the shoretel system) dial a telephone number and have a call continue past the 20 minute mark. I usually abandon the call at the 20 min mark. Pick up a Shoretel IP480G telephone set, Dial a telephone number, place the call on hold then unhold, the call continues past the 15:36 mark. At the 15:36 mark, the far end drops but the trunk test tool and communicator (I have not looked at the icon on the telephone set yet) show the call still in progress. Pick up a shoretel IP480G set and dial a telephone number. Outgoing calls placed on SIP trunks disconnect at fifteen minutes and thirty six seconds.
