SIP 503 Service Unavailable
AsteriskRFC 3261 §21.5.4The server is temporarily unable to process the request. Often emitted by SBCs / proxies under load or when the next-hop is down.
Common causes on Asterisk
- Trunk to next-hop is down (carrier outage)
- SBC overloaded / hit a quota
- DNS SRV failover in progress
- Asterisk PJSIP `qualify` marked endpoint unreachable
Asterisk-specific note
If `qualify_frequency` marked the endpoint unreachable, you'll see 503. `pjsip show endpoint <name>` will show its status.
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