Causas Sip

21. Response Codes
The response codes are consistent with, and extend, HTTP/1.1 response
codes. Not all HTTP/1.1 response codes are appropriate, and only
those that are appropriate are given here. Other HTTP/1.1 response
codes SHOULD NOT be used. Also, SIP defines a new class, 6xx.
21.1. Provisional 1xx
Provisional responses, also known as informational responses,indicate that the server contacted is performing some further action
and does not yet have a definitive response. A server sends a 1xx
response if it expects to take more than 200 ms to obtain a final
response. Note that 1xx responses are not transmitted reliably.
They never cause the client to send an ACK. Provisional (1xx)
responses MAY contain message bodies, includingsession descriptions.
21.1.1. 100 Trying
This response indicates that the request has been received by the
next-hop server and that some unspecified action is being taken on
behalf of this call (for example, a database is being consulted).
This response, like all other provisional responses, stops
retransmissions of an INVITE by a UAC. The 100 (Trying) response is
differentfrom other provisional responses, in that it is never
forwarded upstream by a stateful proxy.
21.1.2. 180 Ringing
The UA receiving the INVITE is trying to alert the user. This
response MAY be used to initiate local ringback.
21.1.3. 181 Call Is Being Forwarded
A server MAY use this status code to indicate that the call is being
forwarded to a different set of destinations.21.1.4. 182 Queued
The called party is temporarily unavailable, but the server has
decided to queue the call rather than reject it. When the callee
becomes available, it will return the appropriate final status
response. The reason phrase MAY give further details about the
status of the call, for example, "5 calls queued; expected waiting
time is 15 minutes". The server MAYissue several 182 (Queued)
responses to update the caller about the status of the queued call.
21.1.5. 183 Session Progress
The 183 (Session Progress) response is used to convey information
about the progress of the call that is not otherwise classified. The
Reason-Phrase, header fields, or message body MAY be used to convey
more details about the call progress.
21.2.Successful 2xx
The request was successful.
21.2.1. 200 OK
The request has succeeded. The information returned with the
response depends on the method used in the request.
21.3. Redirection 3xx
3xx responses give information about the user's new location, or
about alternative services that might be able to satisfy the call.
21.3.1. 300 Multiple Choices
The address in therequest resolved to several choices, each with its
own specific location, and the user (or UA) can select a preferred
communication end point and redirect its request to that location.

The response MAY include a message body containing a list of resource
characteristics and location(s) from which the user or UA can choose
the one most appropriate, if allowed by the Acceptrequest header
field. However, no MIME types have been defined for this message
body.

The choices SHOULD also be listed as Contact fields (Section 20.10).
Unlike HTTP, the SIP response MAY contain several Contact fields or a
list of addresses in a Contact field. UAs MAY use the Contact header
field value for automatic redirection or MAY ask the user to confirm
a choice.However, this specification does not define any standard
for such automatic selection.

This status response is appropriate if the callee can be reached
at several different locations and the server cannot or prefers
not to proxy the request.
21.3.2. 301 Moved Permanently
The user can no longer be found at the address in the Request-URI,
and the requesting...