SIP 503 Service Unavailable
TwilioRFC 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 Twilio
- Trunk to next-hop is down (carrier outage)
- SBC overloaded / hit a quota
- DNS SRV failover in progress
- Asterisk PJSIP `qualify` marked endpoint unreachable
Stop guessing — paste your trace
Drop your PCAP or sngrep log and SIP Flow will render the ladder, surface the failing transaction, and (optionally) ask Gemini to suggest the fix grounded in Twilio docs.
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