[Explanation]
Extension 201 does not answer the trunk call, so the call is forwarded to the Unified Messaging system and
answered by mailbox 201. If the outside caller leaves a message, the Unified Messaging system sends a
Message Waiting notification to the extension using Enhanced QSIG information over the private network.
When forwarding the call, PBX-2 sends any received call information (Caller ID number/name, DDI number)
along with the trunk group number to use, applicable time mode, and extension number and forwarding reason
of the original destination extension to PBX-1 as Enhanced QSIG information.
410 Feature Guide
4.3.5 QSIG Enhanced Features