I have experienced intermittent issues with my internet SIP trunk provider.  Users can join conferences if they join online and input a call back number or join over the PC.  Unfortunately they cannot call in to the DIDs for the dialin conferencing.  I do not see any error in the lync monitoring reports at all.  I do however see the following error in my error logs on my Front End.

Event 46009, LS Outbound Routing

An attempt to route to a gateway failed.

Could not route to gateway x.x.x.x, the attempt failed with response code: 504 server time-out (callId:  xxxx).

Failure occurences: 2, since 6/24/2013 8:02:10 AM.

Cause: A gateway failed ro respond to a request within allotted time or was unable to route the request due to some error.

Resolution: Check whether the spcefified gateway is up and is properly configured.

How do I go about troubleshooting this?  I checked my firewall to verify the traffic wasn't being blocked and checked the AV on the front end to ensure it wasn't blocking lync traffic.  My Internet SIP trunk provider was not receiving responses from my front end.  All of a sudden it started working again with absolutely no changes made by me or Internet sip trunk provider(Intelepeer).  I am at a loss.  Intelepeer told me it was an issue on my end but offered to test whenever I like.  Any suggestions? 

I did finally see issues in the Lync Monitoring Server.  here is what I found when viewing the diagnostic report.  There were a number of successes then one failure.

Diagnostic header: 

10504; source="frontend.domain.com"; reason="Gateway responded
with 504 Server Time out"; component="MediationServer"; SipResponseCode="504";
SipResponseText="Server Time-out"; sip-reason="Q.850; cause=102";
GatewayFqdn="x.x.x.x"

Additional Diagnostic information:

It seems related then network traffic. You should check whether there is data package loss or not.

Please also check if this issue happened randomly or to specific user.

If it happened with specific user, you should check that this use used the correct route, PSTN usage.