In a SIP protocol, there could be a variety of response codes during a call. They are primarily categorised into six classes –

• 1xx is ‘Informational’ or ‘provisional’. These codes are noticed before a connection is made.
• 2xx is ‘Success’
• 3xx is a ‘Redirection’
• 4xx is a ‘Client Error’
• 5xx is a ‘Server Error’
• 6xx is a ‘Global Failure’

Some of the most common codes for each of the six classes are below –

1xx – Informational

A 1xx SIP response code can be sent at any time while a connection is being established. Some common 1xx codes are:

• 100 – Trying: The request has been received, and an unspecified action is being taken on behalf of the call. 

• 180 – Ringing: An invite has been received by the user agent and it is now trying to alert the user. 

• 181 – Call is Being Forwarded: Indicates that the call is being forwarded to another endpoint.

• 183 – Session Progress: This code is used to give information about the progress of a call. 

• 199 – Early Dialog Terminated: Indicates to upstream SIP entities that an early dialog has been terminated. 

2xx – Success

The 2xx response codes are used to indicate that a SIP request has been successfully received, understood, and accepted. You’ll typically see the following versions:

• 200 – Ok: The request has succeeded.

• 202 – Accepted: The UAS has received and understood the request, but it may not have been authorized or processed by the server. 

• 204 – No Notification: The request was successful, but the response will not be received.

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: The request address returned several choices, each with their own specific locations. In this case, the UA can select a preferred endpoint to redirect the request to that specific location.

• 301 – Moved Permanently: The user can no longer be found at the address used in the request. A new address will be given in the contact header field, which can be retried by the requesting client. The new address should be saved and used in all future invite requests. 

• 302 – Moved Temporarily: A new address will be given in the contact header field, which can be retried by the requesting client. This address should not be saved for future invite requests. 

• 305 – Use Proxy: A proxy must be used to access the required destination. The specified proxy will be displayed in the contact field. 

• 380 – Alternate Service: The call failed, but the message body details alternatives. 

4xx – Client Error

The 4xx response codes indicate that something went wrong while processing the message, and the request cannot be fulfilled. There are quite a few of them, including:

• 400 – Bad Request: The request could not be understood. 

• 401 – Not Authorized: The request requires user authentication.

• 404 – Not Found: The server has definitive information that the user does not exist at that particular domain. 

• 407 – Proxy Authentication Required: This code is similar to the 401 – Not Authorized code, but in this case, the client must authenticate itself with the proxy. 

• 408 – Request Timeout: The server could not produce a response within a suitable time frame. 

• 415 – Unsupported Media Type: The server is refusing to service the request because the message body is in a format not supported by the server for the particular request method. 

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

The server was prevented from fulfilling the request by an unexpected condition. 

• 501- Not Implemented

The functionality required to fulfill the request is not supported by the server.

• 502 – Bad Gateway

When attempting to fulfill the request, the server received an invalid response from a downstream server. 

• 503 – Service Unavailable

Temporary overloading or maintenance of the server means it is currently unable to process the request. The client should attempt to forward the request to another server.

• 504 – Server Time-Out

When attempting to process the request, the server did not receive a timely response form the external server.

6xx – Global Error

Finally, the 6xx response codes relate to Global Error issues. They include:

• 600 – Busy Everywhere: The callee’s end system was contacted successfully but the callee is busy. 

• 603 – Decline: The callee’s end system was successfully contacted but the user does not wish to or cannot participate. 

• 604 – Does Not Exist Anywhere: There is information that the user indicated in the request URI does not exist. 

• 606 – Not Acceptable: The user wishes to communicate but they cannot adequately support the session described.

This above info was sourced from https://telnyx.com/resources/sip-response-codes-need-know-2-minutes.