401 4xx Client error RFC 3261
SIP 401 Unauthorized
The request needs user authentication. Sent by registrars and user agents with a WWW-Authenticate challenge. The client should resend with credentials.
Typical Q.850 / ISDN cause: 21 (Call rejected)
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Common causes
- Normal first step of digest authentication on REGISTER.
- Wrong username, password, or auth username (auth ID).
- Realm mismatch between client and server.
How to fix it
- One 401 followed by success is normal. Repeated 401s mean the credentials are wrong.
- Check the auth username separately from the extension number. Many systems use different values.
- Verify realm and clock or nonce settings. Look at the server logs for the reason the digest was rejected.
Seeing 401 on calls through a router or firewall? Check that SIP ALG is disabled. It causes a surprising number of SIP errors. Read why.
About 4xx responses
The request has a problem or cannot be completed at this server, usually because of something on the requesting side or the target user.