SIP 421 Extension Required
What SIP 421 means
The mirror image of a 420: the server needs an extension your device did not offer, and names it in the response. Enable the named capability and the request succeeds.
What SIP 421 Extension Required actually means
The server needs a particular protocol extension to process the request and the request did not indicate support for it, so the response carries a Require header naming what is needed. It is a request for a capability rather than a refusal of one.
What causes SIP 421 on a business phone system
- The platform requires a capability the device has disabledCommonly session timers or reliable provisional responses, switched off during earlier troubleshooting and never restored.
How to fix SIP 421
- Read the Require header and enable that capabilityThe response names precisely what to switch on, so this is usually a single setting.
- Check what earlier troubleshooting disabledWhere a capability was turned off to work around a different problem, confirm that problem still exists before leaving it off.
Fixing the underlying problem
This page explains the code. These guides walk through the fix in detail.
Questions about SIP 421
What is the difference between SIP 420 and SIP 421?
They are opposite ends of the same negotiation. A 420 means you insisted on an extension the server does not understand, so you should relax it. A 421 means the server needs an extension you did not offer, so you should enable it. In both cases the response names the extension.
Related SIP response codes
Source. SIP 421 Extension Required is defined in RFC 3261 section 21.4.16 and registered in the IANA SIP Parameters — Response Codes. The causes and fixes above are drawn from our own experience supporting UK business phone systems.