SIP 491 Request Pending
KamailioRFC 3261 §14.1A re-INVITE collided with a pending request. RFC 3261 prescribes a backoff retry; usually transient.
Common causes on Kamailio
- Glare from simultaneous re-INVITEs (hold/unhold, codec change)
- Some SIP-T/SIP-I gateways re-INVITE aggressively
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 Kamailio docs.
Open analyzerDon't have a trace yet? See how to capture one in Wireshark, sngrep, or your PBX