421
SIP
421 Extension Required
The UAS needs a particular extension to process the request but the extension is not listed in the Supported header.
On this page
When You See This Code
When the server requires a specific SIP extension that the client did not indicate support for.
How to Fix
Add the required extension to the Supported header and ensure your UA implements it. Check the response for which extension is needed.
Specification
Compare With
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
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PBX (Private Branch Exchange)
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TLS Versions
Frequently Asked Questions
What is SIP 421 Extension Required?
The UAS needs a particular extension to process the request but the extension is not listed in the Supported header.
When do you see SIP 421 Extension Required?
When the server requires a specific SIP extension that the client did not indicate support for.
How to fix SIP 421 Extension Required?
Add the required extension to the Supported header and ensure your UA implements it. Check the response for which extension is needed.