Define a gateway in Topology Builder in Skype for Business Server

Summary: Learn how to define a PSTN gateway in Topology Builder in Skype for Business Server.

Follow these steps to use Topology Builder to define a peer with which you can associate a Mediation Server to provide connectivity to the public switched telephone network (PSTN) for users enabled for Enterprise Voice. A peer to the Mediation Server can be a PSTN gateway, an IP-PBX, or a Session Border Controller (SBC) for an Internet Telephony Service Provider (ITSP) to which you connect by configuring a SIP trunk.

To define a peer for the Mediation Server

  1. Start Topology Builder: Click Start, click All Programs, click Skype for Business Server 2015, and then click Skype for Business Server 2015Topology Builder.

  2. Under Skype for Business Server, your site name, Shared Components, right-click the PSTN Gateways node, and then click New PSTN Gateway.

  3. In Define New IP/PSTN Gateway, type the fully qualified domain name (FQDN) or IP address of the peer, and click Next.


    If you specify Transport Layer Security (TLS) as the transport type, you must specify the FQDN instead of the IP address of the peer of the Mediation Server.

  4. Define the listening mode (IPv4 or IPv6) of the IP address of your new PSTN gateway, and click Next.

  5. Define a root trunk for the PSTN gateway. A trunk is a logical connection between a Mediation Server and a gateway uniquely identified by the tuple.

    {Mediation Server FQDN, Mediation Server listening port (TLS or TCP) : gateway IP and FQDN, gateway listening port}

    • When defining a PSTN gateway in Topology Builder, you must define a root trunk to successfully add the PSTN gateway to your topology.

    • The root trunk cannot be removed until the associated PSTN gateway is removed.

  6. Under Listening Port for IP/PSTN Gateway, type the listening port that the gateway, PBX, or SBC will use for SIP messages from the Mediation Server that will be associated with the root trunk of the PSTN gateway. (By default, the ports are 5066 for Transmission Control Protocol (TCP) and 5067 for Transport Layer Security (TLS) on a PSTN gateway, PBX or SBC. On a Survivable Branch Appliance at a branch site, the default ports are 5081 for TCP and 5082 for TLS.)

  7. Under SIP Transport Protocol, click the transport type that the peer uses, and then click OK.


    For security reasons, we strongly recommend that you deploy a peer to the Mediation Server that can use TLS.

  8. Under Associated Mediation Server, select the Mediation Server pool to associate with the root trunk of this PSTN Gateway.

  9. Under Associated Mediation Server port, type the listening port that the Mediation Server will use for SIP messages from the gateway.


    With multiple trunk support in Skype for Business Server, you can define multiple SIP signaling ports on the Mediation Server for communication with multiple PSTN gateways. When defining a trunk, the Associated Mediation Server port must be within the range of the listening ports for the respective protocol allowed by the Mediation Server. This port range is defined under Skype for Business Server and Mediation Pools. Right-click the Mediation Server pool of interest, and select Edit Properties. Specify the port range in the Listening ports field.

  10. Be sure that the peer you defined is running and using the FQDN or IP address that you specified. Then click Finish.

  11. Right-click the Skype for Business Server node, and then click Publish Topology.