I was working on troubleshooting a Lync Conferencing issue today and found that I was getting a busy signal when calling into the conference bridge number.
I first confirmed the mediation ports responded to opening them on 5060, then progressed to log the call to dig into it with Snooper.
I found the following error, after the call was received by the mediation server:
"Lync SIP/2.0 504 Server time-out".
It looked like the mediation server was having a problem connecting to the audio conferencing service on 5072. A "netstat -a" confirmed this. I was curious, as I saw several of the conferencing services on the front end and we had used it on this server before. I also confirmed that the PSTN conferencing was selected on the topology builder.
In looking at the services I noticed that the "Lync Server Conferencing Attendant" and "Lync Server Conferencing Announcement" services were missing!! I then proceeded to re-run the Lync Server Deployment Wizard again, after which the services were deployed and calls completed successfully.
The key point is that in troubleshooting Lync, as well as many other things, it helps to understand the ports and traffic flow to quickly determine the cause of a problem. I highly recommend taking a look at the Lync traffic flow diagram here:
I've also posted a quick reference to the traffic flow diagrams here: