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ProtocolCodes
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.

On this page

When You See This Code

When sending a re-INVITE while another re-INVITE is still being processed. Common during hold/unhold or codec change sequences.

How to Fix

Wait for the pending transaction to complete before sending a new one. Implement proper glare handling (RFC 3261 Section 14.1).

Specification

View RFC / Specification →

Compare With

100 Trying 180 Ringing 181 Call Is Being Forwarded 182 Queued 183 Session Progress 199 Early Dialog Terminated 200 OK 202 Accepted 204 No Notification 300 Multiple Choices 301 Moved Permanently 302 Moved Temporarily

Guides

401 Unauthorized vs 403 Forbidden: When to Use Each Security & Authentication
SIP Response Codes: Understanding VoIP Call Flow Real-Time Protocols
SIP Call Flow: Understanding Session Setup Real-Time Protocols
WebRTC Signaling with SIP and WebSocket Real-Time Protocols
RFC 3261: SIP Protocol Deep Dive Protocol Deep Dives
TLS 1.3: Faster, Simpler, More Secure Protocol Deep Dives

Glossary

Status Code Request Method (HTTP Verb) TLS Handshake Authentication TCP/IP Port Number NAT (Network Address Translation) Socket Request-Response Model RFC (Request for Comments) Connection-Oriented Full-Duplex SRV Record SIP Dialog Signaling SIP Registrar STUN (Session Traversal Utilities for NAT) TURN (Traversal Using Relays around NAT) SDP (Session Description Protocol) SIP INVITE SIP Proxy Server SIP Redirect Server SIP Transaction SIP User Agent SIP Trunk RTP (Real-time Transport Protocol) SRTP (Secure Real-time Transport Protocol) SIP ACK SIP BYE SIP CANCEL SIP OPTIONS SIP SUBSCRIBE/NOTIFY B2BUA (Back-to-Back User Agent) SBC (Session Border Controller) Codec Negotiation SIP Response Classes VoIP (Voice over IP) Presence (SIP) PBX (Private Branch Exchange) MFA (Multi-Factor Authentication) TLS Versions

Frequently Asked Questions

What is 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.

When do you see SIP 491 Request Pending?

When sending a re-INVITE while another re-INVITE is still being processed. Common during hold/unhold or codec change sequences.

How to fix SIP 491 Request Pending?

Wait for the pending transaction to complete before sending a new one. Implement proper glare handling (RFC 3261 Section 14.1).