Audio calls via a Lync Server 2013 Edge server gets stuck at connecting with no audio for 10 seconds then disconnects

I was recently asked by a client to troubleshoot an issue with their Lync Server 2013 environment where users connecting remotely via the Edge server are unable to establish voice calls. A user would be able to successfully authenticate as well as see the call come in but when they pick up with their handset, they would not hear any audio and the call would disconnect within 10 seconds. Using the Skype for Business client would show the call in the Connecting call… status then disconnect:

While this type of issue can be caused by many reasons, this environment I had to troubleshoot the issue in had 3 contributing misconfigurations:

#1 – A/V Edge service NAT was not configured

The Edge server’s external interface IPs were all NAT-ed but the topology for the environment did not have the external IP address configured:

#2 – The DNS record of the Edge server’s internal interface was not correct

The Edge server for this environment was recently moved with the internal interface’s IP address was changed and since the server is not joined to the domain, the internal DNS A record which servers such as the front-end server uses to reach the Edge server was not updated.

#3 – Port 5062 was not opened between the Edge server and FE / SBA servers

The internal interface of the Edge server was in a secured network and TCP port 5062 used for authentication of A/V users was not opened. The following TechNet article provides more detail about the purpose of this port:

Edge Server environmental requirements in Skype for Business Server
https://docs.microsoft.com/en-us/skypeforbusiness/plan-your-deployment/edge-server-deployments/edge-environmental-requirements

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Note that other than discovering audio does not work, the Remote Connectivity Analyzer tool that Microsoft provides can help with identifying such an issue: https://testconnectivity.microsoft.com

The following is the output from a test ran against the environment:

Testing remote connectivity for user tluk@domain.com to the Microsoft Lync server.
      Specified remote connectivity test(s) to Microsoft Lync server failed. See details below for specific failure reasons.
        Tell me more about this issue and how to resolve it

     
Additional Details
      Couldn’t sign in. Error: User failed to get response from MRAS server.
SIP service request to MRAS server failed.
Error Message: A 504 (Server time-out) response was received from the network and the operation failed. See the exception details for more information..
Error Type: PublishSubscribeException.
Fault Code: .
Response Code: 504.
Response Text: Server time-out.
Diagnostic Header: ErrorCode=1038,Source=UKSBA03.contoso.COM,Reason=Failed to connect to a peer server,fqdn=ukles03.contoso.com:5062,peer-type=InternalServer,winsock-code=10060,ip-address=192.168.34.10,winsock-info=The peer did not respond to the connection attempt
Microsoft.Rtc.Signaling.DiagnosticHeader

Elapsed Time: 12486 ms.