Fault & error code checker

Fault and error codes rarely explain themselves. Enter one below to get its official meaning, what causes it on a UK business system, and how to fix it.

Most looked-up codes

• SIP 403ForbiddenYour credentials were accepted but the action was not allowed. A 403 means the system knows…
• SIP 480Temporarily UnavailableThe extension exists but nothing is available to ring. On a hosted system this nearly always…
• SMTP error 5.7.1Delivery not authorized, message refusedSomebody decided you are not permitted to send to that recipient, and refused the message outright.…
• SMTP error 5.1.1Bad destination mailbox addressThere is no mailbox by that name on the receiving system. The part to the left…
• SIP 401UnauthorizedA challenge for credentials, not a failure. One 401 followed by a successful registration is how…
• Q.850 cause 16Normal call clearingNot a fault. Cause 16 means somebody hung up — it is the normal ending for…
• SMTP error 5.7.8Authentication credentials invalidThe username and password presented were rejected, so nothing was ever submitted. It appears as 535…
• SMTP error 5.7.23SPF validation failedThe receiving server looked up the SPF record for your domain, compared it against the IP…

Browse by device or protocol

• VoIP & SIP40 SIP response codes explained in full
• Call hangup causes (Q.850)38 Q.850 cause values explained in full
• Email delivery & SMTP48 SMTP status codes explained in full

How this works

Every code here is checked against the official registry that defines it, and each page links to the exact specification section so you can verify it. What the registry cannot tell you is why your phone system is producing the code, so that part — the causes and the fixes — comes from supporting UK business phone systems rather than from the standard.

Codes we have not researched properly are still listed on each family page with the document that defines them, but they do not get a page of their own. We would rather show you the registry entry and admit we have nothing to add than pad out a page that tells you nothing.

Looking for router settings instead?

If you need a gateway address, a default username or the reset procedure for a specific make of router, that is a different question and it has its own tool: the router checker covers admin addresses, default credentials and status light meanings by model.