Call looping can occur between a Microsoft Lync Server VoIP deployment and the VoIP gateway that is used to route outgoing calls in the following scenario:
A group of Direct Inward Dialing (DID) telephone numbers are designated for Lync Server Enterprise Voice (EV)-enabled users.
Some of these DID telephone numbers will be unassigned and available for the future use of Lync Server EV-enabled users.
A Lync user or PSTN user dials one of the unassigned DID telephone numbers.
Lync Server performs a reverse number lookup (RNL) on the unassigned DID telephone number and detects that the telephone number is not assigned to a Lync Server EV-enabled user. The call is then routed to the VoIP gateway as an outgoing VoIP call.
The VoIP gateway recognizes the called number as a DID telephone number that was provisioned for the Lync Server VoIP deployment and routes a request for a new call to the Lync Server VoIP deployment.
The Lync Server VoIP deployment again performs an RNL on the unassigned DID number and sees that the number is not assigned to a Lync Server EV-enabled user. Again, the call is routed to the VoIP gateway as an outgoing call.
In this scenario, the dialed number might be processed in a continuous loop between the Lync Server VoIP deployment and the VoIP gateway. This call-looping behavior can result in a lack of available Windows Server TCP source port addresses that will be needed by the Windows Server for future outgoing TCP connections.
The call-looping problem that is described in the "Summary" section can occur when the Lync Server deployment is not configured to use a Lync Server unassigned number table to manage the routing of VoIP calls to an announcement service. For more information about how to configure an unassigned number table for use with Lync Server VoIP deployments, go to the following Microsoft TechNet website: