Knowledgebase

4xx status codes are defined in RFC 3261 and its successors.

They indicate the request was malformed or otherwise cannot be fulfilled due to client-side errors.

They do not indicate server failure. Instead, they’re triggered when a SIP device, endpoint, or intermediary proxy receives something it cannot or will not accept based on the SIP protocol or local policies.

Here’s a breakdown of each code, in numerical order, with correct definitions, practical examples, and deeper protocol insights.

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400 – Bad Request

• The request could not be understood due to malformed syntax.

• Could be caused by incorrect header formatting, missing fields (e.g. To, From, Call-ID), or malformed URI.

• Example: INVITE sent with an improperly encoded SDP body.

401 – Unauthorized

• Indicates that authentication is required to proceed.

• Used mainly by registrars and UASs to challenge the request.

• Includes a WWW-Authenticate header with nonce values and realm.

402 – Payment Required

• Reserved for future use (not implemented in SIP today).

• Might be used in future billing or prepaid systems.

403 – Forbidden

• Server understood the request but refuses to fulfil it.

• Not a challenge; re-sending credentials will not help.

• May be returned if the source IP or username is banned or blocked.

404 – Not Found

• The user is not known at the domain in the Request-URI.

• Returned by registrars or proxies unable to locate the target.

405 – Method Not Allowed

• The method (e.g. OPTIONS, INVITE) is known but not permitted for the requested resource.

406 – Not Acceptable

• Server cannot return a response acceptable to the client.

• Often relates to media types and Accept: headers.

407 – Proxy Authentication Required

• Same as 401, but issued by a SIP proxy, not the UAS.

• Requires a valid Proxy-Authenticate header to be processed.

408 – Request Timeout

• The server couldn’t find the target user in time.

• Often caused by NAT issues, DNS failures, or dropped packets.

• May be retried.

409 – Conflict (Deprecated)

• Previously used when a user was already registered.

• Removed in newer RFCs; not used in modern SIP implementations.

410 – Gone

• The user used to exist but no longer does at this location.

• Unlike 404, this is permanent.

411 – Length Required (Deprecated)

• The request must include a valid Content-Length.

• Obsolete in SIP since all requests should already include this.

412 – Conditional Request Failed

• A required condition for the request has not been met.

• Rare in practice.

413 – Request Entity Too Large

• The request body exceeds server limits.

• May be triggered by over-sized SDP or headers.

414 – Request-URI Too Long

• The URI provided in the request is too long for the server to process.

415 – Unsupported Media Type

• The server does not support the format in the message body.

• Could be an unsupported SDP codec or an invalid content type.

416 – Unsupported URI Scheme

• The URI scheme is unknown or not allowed (e.g. "mailto:" instead of "sip:").

417 – Unknown Resource-Priority

• The Resource-Priority header contained a tag that the server doesn’t understand.

420 – Bad Extension

• The request uses an unsupported SIP extension.

• Look for a Require: header with unknown options.

421 – Extension Required

• The server needs an extension not supported by the requestor.

• The required extensions are listed in the Require: header.

422 – Session Interval Too Small

• The Session-Expires value is below the server’s minimum acceptable time.

423 – Interval Too Brief

• The Expires: or similar timer value is too short.

424 – Bad Location Information

• Provided location data is malformed or incomplete.

425 – Bad Alert Message

• The emergency call alert was malformed enough that no safe response could be initiated.

428 – Use Identity Header

• The provider requires an Identity: header and it was not included.

429 – Provide Referrer Identity

• The request lacked a valid Referred-By: token.

430 – Flow Failed

• One flow to the UA failed (seen in Outbound RFC 5626 scenarios). (and if it is seen by one, should be treated as a 400 Bad Request response)

433 – Anonymity Disallowed

• Caller’s identity is hidden but anonymity is not allowed by recipient policy.

436 – Bad Identity-Info

• The URI in the Identity-Info: header can’t be resolved.

437 – Unsupported Certificate

• Server can’t validate the provided certificate.

438 – Invalid Identity Header

• The Identity: signature cannot be verified.

439 – First Hop Lacks Outbound Support

• The initial SIP proxy does not support outbound registrations, which the registrar requires.

440 – Max-Breadth Exceeded

• Forking breadth (parallel target reach) exceeded. No fallback method provided.

469 – Bad Info Package

• The INFO request refers to an unsupported Info-Package.

470 – Consent Needed

• The source lacks permission to initiate this request.

480 – Temporarily Unavailable

• Callee cannot currently be reached; no forwarding is available.

481 – Call/Transaction Does Not Exist

• No active dialog or transaction matched the incoming request.

482 – Loop Detected

• SIP message is stuck in a routing loop.

483 – Too Many Hops

• Max-Forwards: header reached zero.

484 – Address Incomplete

• The URI lacks digits or format (e.g. partial number).

485 – Ambiguous

• Address is ambiguous (multiple matches found).

486 – Busy Here

• Callee is busy and declines the call.

487 – Request Terminated

• The request (usually INVITE) was cancelled by a CANCEL or BYE.

488 – Not Acceptable Here

• Session parameters (media, transport, etc.) aren’t supported by the target.

489 – Bad Event

• Event package in the Event: header is unknown or unsupported.

491 – Request Pending

• Another request is in progress in this dialog.

493 – Undecipherable

• Message body is encrypted but recipient can’t decrypt it.

494 – Security Agreement Required

• Secure communication required but not negotiated. Response will contain supported methods.

Why this matters

If your provider:

• Doesn’t explain what went wrong

• Sends incomplete SIP responses

• Doesn’t help you troubleshoot issues like 408s or 488s

…then you’re left guessing.

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Let’s face it: if you're seeing SIP 4xx codes often, your current setup may not be fully compliant or stable.

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Last updated: 01/04/2025

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