420
SIP
420 Bad Extension
The server did not understand the protocol extension specified in a Require or Proxy-Require header. The Unsupported header lists the unknown extensions.
متى ترى هذا الكود
When the request includes a Require header with SIP extensions that the server does not support.
كيفية الإصلاح
Check the Unsupported header in the response. Remove the unsupported extension from the Require header or make it optional via Supported.