Informational responses are used to indicate call progress. Normally the responses are end to end (except 100 Trying).

This class of responses is meant for indicating that a request has been accepted.

Generally these class responses are sent by redirect servers in response to INVITE. They are also known as redirect class responses.

Client error responses indicate that the request cannot be fulfilled as some errors are identified from the UAC side.

This class response is used to indicate that the request cannot be processed because of an error with the server.

This response class indicates that the server knows that the request will fail wherever it is tried. As a result, the request should not be sent to other locations.