SIP 491 Request Pending

Cisco CUBERFC 3261 §14.1

A re-INVITE collided with a pending request. RFC 3261 prescribes a backoff retry; usually transient.

Common causes on Cisco CUBE

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 Cisco CUBE docs.

Open analyzer

Don't have a trace yet? See how to capture one in Wireshark, sngrep, or your PBX

Same error on a different vendor

491 on Asterisk
491 on FreeSWITCH
491 on Kamailio
491 on OpenSIPS
491 on Twilio
491 on Generic SIP