SIP
504 Server Time-out
The server did not receive a timely response from an external server it accessed while attempting to process the request. Different from 408 (client timeout).
When You See This Code
When a proxy's request to a downstream server times out. The proxy itself is working, but the next hop is unresponsive.
How to Fix
Check the downstream server's availability. Verify DNS resolution for the next-hop domain. Consider adjusting transaction timeout values.