SIP Response Codes: All You Need to Know in Under 2-Minutes
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SIP Response Codes: All You Need to Know in Under 2-Minutes

If you’re unaware, SIP is essentially a call and response protocol. If you send a message, you’ll get a response, and in some cases, you might get multiple responses—lucky you! For the sake of adding a little deeper context from a technical perspective, a SIP response will contain information sent by a User Agent Server (UAS) to a User Agent Client (UAC).
These responses are specially created to help and inform, with the range of codes that you could potentially receive being split into six types. Each type has its own three-digit code, with the first digit of the code identifying the class type of the response:
  • 1xx is ‘Informational’
  • 2xx is ‘Success’
  • 3xx is a ‘Redirection’
  • 4xx is a ‘Client Error’
  • 5xx is a ‘Server Error’
  • 6xx is a ‘Global Failure’
These types of codes can also be broken into two categories: code 1xx is known as a ‘Provisional’ code, and codes 2xx-6xx are referred to as ‘Final.’
The easy way to remember these is that a ‘Provisional’ code can be sent up until the point of establishment, or before you’ve made a connection. The ‘Final’ codes will be sent to confirm whether you’ve successfully made a connection or not (error, redirection, failure)

Which SIP response code should I remember?

Each code will be accompanied by an English text format to help explain it, so thankfully you don’t necessarily need to remember all the codes and what they stand for. It might help to know what action to take based on what you receive back from the UAS. Below are the most common types of SIP response code that you’re likely to come across with each serving their own unique purpose.

1xx - Informational

As mentioned earlier, a 1xx SIP response code can be sent at anytime while a connection is being established. Some of the regularly received codes are:
  • 100 - Trying
  • 180 - Ringing
  • 181 - Call is Being Forwarded
  • 183 - Session in Progress
  • 199 - Early Dialog Terminated

2xx - Success

The 2xx response codes are used to indicate that a SIP request has been successfully processed. You’ll typically see the following versions:
  • 200 - Ok
  • 202 - Accepted
  • 204 - No Notification

3xx - Redirection

These notify the UAC on redirections and of the different routes available to get to the UAS. Commonly received examples include:
  • 300 - Multiple Choices
  • 301 - Moved Permanently
  • 302 - Moved Temporarily
  • 305 - Use Proxy
  • 380 - Alternate Service

4xx - Client Error

The 4xx response codes are used to indicate that something went wrong while processing the message - and there are quite a few of them, including, but not limited to: - 400 - Bad Request - 401 - Unauthorized - 404 - Not Found - 407 - Proxy Authentication Required - 408 - Request Timeout - 415 - Unsupported Media Type

5xx - Server Error

5xx responses relate to server error issues and are mostly generated by the likes of proxy servers, location servers, and redirect servers. You’ll be familiar with some of these: - 500 Server Internal Error - 501 Not Implemented - 502 Bad Gateway - 503 Service Unavailable - 504 Server Time-Out

6xx - Global Error

Finally, the 6xx response codes relate to Global Error issues. They include: - 600 Busy Everywhere - 603 Decline - 604 Does Not Exist Anywhere - 606 Not Acceptable

About Telnyx
Telnyx delivers voice, messaging and more for applications and next-generation communications companies. Our elastic SIP trunking scales on demand, allowing you to instantly provision connectivity through an easy-to-use platform and API, and our global infrastructure ensures crystal clear calls every time. Contact our customer success team to learn more.
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