I have Lync set up but I cant contact someone on an external network I get this error 

When contacting your support team, reference error ID 504   (source ID 239). Will you be at the event in Person

are you trying to connect to users in your organization or from other organization. 

This is a common error with federation. If it is federated users, were they ever working? If not have you configured the edge server and the federation settings? Have you enabled users to be able to federate?

Hi Somet,

Agree with Georg. Would you elaborate more about your issue and your Lync environment (especially Edge server and Federation configuration)?

Please check if your Edge server is down.

Please check if the Edge server of your Federated party is down.

If you can receive IMs from someone and get this error when you reply, check their systems external DNS or Edge configuration. Look for wrong or mis-configured DNS or NAT in the Topology.

Please check ALL the certificates in the chain between you and the other end.

Best Regards,

Eason Huang