Each media gateway, IP PBX, or ITSP SIP trunk must be defined as a PSTN gateway within the Lync topology so that it can be referenced by a trunk.
Follow these steps within Topology Builder to define the PSTN gateways and trunks:
1. Expand Shared Components, right-click on PSTN Gateways, and select New IP/PSTN Gateway.
2. Enter the FQDN or IP address for the PSTN gateway and click Next.
3. Select whether to Enable IPv4 or IPv6 for communications to the gateway, and click Next.
4. Enter a Trunk Name, which represents the connection between a Mediation pool and a gateway with a unique port combination.
5. Enter a Listening Port on the IP/PSTN Gateway where calls will be sent.
6. Select whether to use TCP or TLS as the SIP ...