During a migration project I ran into a nasty issue where I was not able to perform any outbound calls via the mediation server and the SIP trunk. As soon as I entered a phone number in my Lync 2013 client and hit the ‘dial’ button the call failed, immediately.

At the same time the following error was logged in the event viewer:

Log Name: Lync Server
Source: LS Outbound Routing
Date: 24-3-2014 13:46:13
Event ID: 46046
Task Category: (1038)
Level: Error
Keywords: Classic
User: N/A
Computer: AMS-LYNCFE01.contoso.com
Description:
A call to a PSTN number failed due to non availability of gateways.
Called Number: +31613123456
Phone Usage: OneXS Interlokaal
Route: OneXS
CallId: a613cdb797de44baa53c4abbbd55029c
Cause: All gateways available for this call are marked as down.
Resolution:
Verify that these gateways are up and can respond to calls.

The Medation server had a good connection with the SIP trunk provider (OneXS) using a fiber backbone connection. I was able to setup a telnet connection on the proper ports. Also connectivity between the Lync 2013 Front-End servers, the Mediation Servers and the Lync 2013 Edge servers was ok. All ports open and routing was functioning as expected.

A few seconds later the following entry was logged in the eventlog:

Log Name: Lync Server
Source: LS Outbound Routing
Date: 24-3-2014 13:46:43
Event ID: 46009
Task Category: (1038)
Level: Error
Keywords: Classic
User: N/A
Computer: AMS-LYNCFE01.contoso.com
Description:
An attempt to route to a gateway failed.
Could not route to Gateway 10.5.82.254, the attempt failed with response code: 503 Service Unavailable (CallID: a613cdb797de44baa53c4abbbd55029c).
Failure occurrences: 1, since 24-3-2014 13:46:13.
Cause: A gateway failed to respond to a request within allotted time or was unable to route the request due to some error.
Resolution:
Check whether the specified gateway is up and is properly configured.

On the first Lync 2013 Edge server I noticed the following entry in the Eventlog:

Log Name: Lync Server
Source: LS Protocol Stack
Date: 24-3-2014 13:45:26
Event ID: 14584
Task Category: (1001)
Level: Warning
Keywords: Classic
User: N/A
Computer: ams-ledge01.contoso.com
Description:
Connection attempt to at least one service in a pool failed.
Connection attempts to the following services have failed. Another attempt will be made for each service every 10 minutes.
Service Address: ams-lyncfe02.contoso.com:5061; Pool FQDN: lync.contoso.com; Down Time: 0:00
Cause: The specified service(s) are unavailable.
Resolution:
Check the servers in the pool(s) on which the service(s) are installed.

After correcting this (after all, the Lync 2013 Edge servers are not domain joined) I checked name resolution from the internal network and it turned out that the Lync 2013 Front-End servers and the Lync 2013 Mediation servers were not able to resolve the Lync Edge Pool FQDN (edge.contoso.com). Added this to the Active Directory DNS, waited for replication, restarted the Lync Server Front-End service on the Front-End server and the Lync Server Mediation service on the Lync 2013 Mediation server and all of a sudden outbound calls were working as expected.

Lessons learned: make sure your name resolution is setup correctly!