SIP response codes: a quick reference to every SIP code and error

What are SIP codes and who sends them?

SIP response codes, or SIP codes, are three-digit status codes a SIP server returns when your PBX, contact center or voice app sends a request such as INVITE or REGISTER. The first digit sets the class, 1xx to 6xx, much like an HTTP status, though not every HTTP code is valid in SIP. "SIP error codes" are the same vocabulary, usually the 4xx to 6xx classes you see when your call fails.

Who sends a SIP response code on your call

A User Agent Client (UAC) sends the request and a User Agent Server (UAS) answers it, so your SIP phone is a UAC when calling and a UAS when answering. A Back-to-Back User Agent (B2BUA), such as a PBX, plays both roles and can rewrite the code it passes to you. As SIP trunking vs VoIP explains, the 503 on your phone may have begun as a 480 upstream.

Provisional versus final SIP responses

1xx codes are provisional and can arrive more than once. Everything from 2xx upward is final and ends the transaction, and for an INVITE you answer the final response with an ACK.

What a SIP response message contains

A response opens with a status line, the SIP version, code and reason phrase, as in SIP/2.0 486 Busy Here. The reason phrase is only a default the sender may reword or translate, so match on the number, never the text. Five headers change what you do next.

Complete list of SIP response codes by class

Each class below carries the codes you will meet on a trunk.

1xx: Informational

A 1xx code tells you the far end has your request and your call is still being set up.

A call that plays an announcement and then fails shows you a 183 first, because 183 carries early media, and a 182 repeats as the queue moves.

2xx: Success

A 2xx code means your request succeeded. 202 Accepted is deprecated in the registry, though older guides still list it for referrals.

3xx: Redirection

A 3xx code means your next INVITE goes somewhere else, and the new address is in the Contact header.

A 301 has no expiry, so update your routing for good.

4xx: Client Error

A 4xx code means the request itself is the problem, usually on your side.

A 401 on your REGISTER is the registrar challenging you, as in SIP registration with Kamailio, while a 407 comes from a proxy. 483 Too Many Hops means Max-Forwards reached 0, usually a routing loop, and an endpoint that sees 430 Flow Failed should treat it as a 400.

5xx: Server Error

A 5xx code means the far end failed, not your request, so the same request may succeed later or on another server.

A 501 means the server does not implement your method at all, while a 405 means it knows the method and disallows it here.

6xx: Global Failure

A 6xx code means your request will fail anywhere, not only at this address.

• 486 vs 600 vs 603: a 486 means only this address is busy, so your PBX may still forward the call. A 600 or 603 means the far end knows there is no alternative such as voicemail.
• 607 vs 608: 607 means a person rejected your call, 608 means an intermediary machine or process did.

SIP response codes reference table

Registered codes run from 100 to 608, and twelve of them cover most of what your trunk returns.

Which SIP code are you looking at?

• Seeing 100, 180 or 183 in the trace: keep waiting, the call is still being set up.
• Got a 200 OK or 202 Accepted: the request worked, so there is nothing to retry or fix.
• Got a 301 or 302 redirect: resend to the address in the Contact header.
• Got a 4xx such as 403 or 404: fix your request, unless it is busy 486, cancelled 487 or unavailable 480.
• Got a 5xx or 6xx such as 503 or 603: wait for any Retry-After before you try again, and treat 603, 607 or 608 as a block.

SIP response codes cheat sheet with the 12 codes to know

These twelve cover most lines in your trunk CDRs, with the move each one calls for.

Full list of SIP codes, 100 to 608

Every registered code in numeric order, including 425 Bad Alert Message, which most lookup pages omit.

Which SIP error codes you will see on live calls, and what to do

Most "SIP error codes" on a healthy trunk are not errors. 486 Busy Here, 487 Request Terminated, 480 Temporarily Unavailable and 603 Decline describe the called party, not your system, and need no fix.

403 Forbidden is often an authorisation or policy failure on your trunk, and 404, 408, 488 and 503 each point at a cause you can check.

Four of the most common codes describe the called party, not your trunk, so only the right column needs action.

Normal call outcomes versus real faults

Each cause and check follows the RFC 3261 meaning.

The carrier-specific meaning of 403 and 503 on a Telnyx trunk is on the Telnyx response codes guide.

When to retry after 302, 408, 503 and 603

RFC 3261 states retry behaviour for a few codes:

• 302: you may cache the result for the Expires period.
• 408 Request Timeout: you may repeat the request unchanged at any later time.
• 503 Service Unavailable and 603: both may carry Retry-After, so wait that long before you try again.
• 404, 486 and 603 Decline: an immediate retry is wasted traffic.

Call blocking and caller identity codes

603 Network Blocked means the network's analytics blocked your call. A 607 Unwanted warns that your next attempt will likely be rejected too. A 608 Rejected should carry a Call-Info header with purpose jwscard and a jCard, so you can dispute the block.

428, 436, 437 and 438 relate to the Identity header used for STIR/SHAKEN. A run of them on your outbound calls means the identity is missing, unreadable or failed verification. Telnyx SIP trunking issues A-level STIR/SHAKEN attestation at the carrier, so your outbound calls go out verified rather than blocked.

Where SIP response codes are defined in RFC 3261 and the IANA registry

The codes come from RFC 3261, the SIP specification, which also created the IANA SIP Parameters registry where every later code is recorded.

RFC 3261 and the RFCs that added codes later

Later RFCs added codes, and old equipment on your trunk can still send codes from RFC 2543.

Reading the IANA SIP Parameters registry

A code absent from the IANA SIP Parameters registry is deprecated.

A response can also carry a Warning header with its own three-digit code, so a 488 with Warning 304 Media type not available tells you which media was refused.

Why SIP codes matter for every call you place

The mix of codes in your CDRs is the fastest read of trunk health. More 503s point at the far end, more 403s or 404s at your configuration, and 603, 607 or 608 at your numbers being treated as spam.

What the codes tell you about your trunk and your carrier

Telnyx runs SIP trunking on a network it owns rather than rents, so you can check a 403 or 503 against the carrier that sent it. A 404 on one of your Telnyx phone numbers means it is not assigned to a connection.

Keeping outbound calls out of 603, 607 and 608

The networks your calls land on set the blocking codes. Telnyx issues STIR/SHAKEN A-level attestation at the carrier, manages CNAM and applies outbound voice profiles against fraud and spam. Usage-based pricing starts at $0.0032 per minute. If you run contact center voice or build voice for ISVs, you place the most exposed volume.

SIP response codes FAQ