What it means
The server is temporarily unable to handle the request, often because of overload or maintenance. It may include a Retry-After header telling you when to try again. This is one of the most common trunk-side rejections.
SIP/2.0 503 Service Unavailable
Common causes
- The server or trunk is overloaded
- Planned maintenance
- Trunk congestion or all circuits busy
- A capacity or licensing limit was hit
- An SBC is rate limiting your traffic
How to fix it
- Honor the Retry-After header if present and back off
- Fail over to a secondary trunk or route
- Check your channel, capacity and licensing limits
- Contact the provider if congestion is on their side
- Spread traffic if you are being rate limited
See it in your own trace
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