SIP
503 Service Unavailable
The server is temporarily unable to process the request due to overloading or maintenance. The client should retry after the time indicated in the Retry-After header.
When You See This Code
During server overload, maintenance windows, or when the server is rate-limiting requests. Common during traffic spikes.
How to Fix
Wait for the time specified in the Retry-After header. If persistent, check server capacity and consider load balancing across multiple SIP proxies.