Recently I was checking into why calls were failing immediately on a specific Lync Mediation Server. I had verified the SIP trunk on the gateway was configured and that the Mediation server could access port 5060 on the gateway's IP address, but couldn't see why the calls were failing. Doing a trace I saw the Mediation Server never even attempt the call, but after restarting the Mediation service I noticed a warning with event ID 25075 logged by LS Mediation Server:
Mediation Server encountered an invalid setting that has been ignored.
Setting: PSTNGatewayService Fqdn: somegatewayname.confusedamused.com
Reason: [Config] A Gateway Peer's (somegatewayname.confusedamused.com) internal settings are not configured correctly: NextHopPort = 5060, TransportType = TCP, NextHopIpAddress = N/A
Cause: Settings configured incorrectly.
Reconfigure the specified setting.
At first glance this appeared to be the correct FQDN for the gateway, but after looking more closely I realized there was a typo in the published topology. The result was the Mediation Server couldn't look up the DNS name for the gateway because it was the wrong name! It's too bad you can't change the FQDN easily in the topology so I had to add a new PSTN gateway with the correct name, add it to all the appropriate routes, and then go back to remove the incorrect name from all routes and the topology. After performing those steps and giving the Mediation service another bounce the calls began to succeed.