491
SIP
491 Request Pending
The request was received by a UAS that had a pending request within the same dialog. The UAS cannot process both requests simultaneously.
Когда вы видите этот код
When sending a re-INVITE while another re-INVITE is still being processed. Common during hold/unhold or codec change sequences.
Как исправить
Wait for the pending transaction to complete before sending a new one. Implement proper glare handling (RFC 3261 Section 14.1).