Quote from themadhacker on April 14, 2021, 3:59 am

VOIP uses SIP messages.

Here are some SIP codes for reference:

SIP Messages

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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 different from
other provisional responses, in that it is never forwarded upstream by a stateful proxy.
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180 Ringing
The UA receiving the INVITE is trying to alert the user. This response MAY be used to
initiate local ringback.
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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.
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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 MAY
issue several 182 (Queued) responses to update the caller about the status of the
queued call.
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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.
Successful 2xx
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The request was successful.
200 OK
The request has succeeded. The information returned with the response depends on the
method used in the request.
Redirection 3xx
3xx responses give information about the user’s new location, or about alternative
services that might be able to satisfy the call.
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300 Multiple Choices
The address in the request 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 Accept request header field. However, no MIME types have been defined
for this message body.
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.
301 Moved Permanently
The user can no longer be found at the address in the Request-URI, and the requesting
client SHOULD retry at the new address given by the Contact header field (Section
20.10). The requestor SHOULD update any local directories, address books, and user
location caches with this new value and redirect future requests to the address(es)
listed.
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302 Moved Temporarily
The requesting client SHOULD retry the request at the new address(es) given by the
Contact header field (Section 20.10). The Request-URI of the new request uses the
value of the Contact header field in the response.
The duration of the validity of the Contact URI can be indicated through an Expires
(Section 20.19) header field or an expires parameter in the Contact header field. Both
proxies and UAs MAY cache this URI for the duration of the expiration time. If there is no
explicit expiration time, the address is only valid once for recursing, and MUST NOT be
cached for future transactions.
If the URI cached from the Contact header field fails, the Request-URI from the
redirected request MAY be tried again a single time.
The temporary URI may have become out-of-date sooner than the expiration time, and a
new temporary URI may be available.
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305 Use Proxy
The requested resource MUST be accessed through the proxy given by the Contact
field. The Contact field gives the URI of the proxy. The recipient is expected to repeat
this single request via the proxy. 305 (Use Proxy) responses MUST only be generated
by UASs.
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380 Alternative Service
The call was not successful, but alternative services are possible.
The alternative services are described in the message body of the response. Formats for
such bodies are not defined here, and may be the subject of future standardization.
Request Failure 4xx
4xx responses are definite failure responses from a particular server. The client
SHOULD NOT retry the same request without modification (for example, adding
appropriate authorization). However, the same request to a different server might be
successful.
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400 Bad Request
The request could not be understood due to malformed syntax. The Reason-Phrase
SHOULD identify the syntax problem in more detail, for example, “Missing Call-ID
header field”.
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401 Unauthorized
The request requires user authentication. This response is issued by UASs and
registrars, while 407 (Proxy Authentication Required) is used by proxy servers.
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402 Payment Required
Reserved for future use.
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403 Forbidden
The server understood the request, but is refusing to fulfill it.
Authorization will not help, and the request SHOULD NOT be repeated.
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404 Not Found
The server has definitive information that the user does not exist at the domain specified
in the Request-URI. This status is also returned if the domain in the Request-URI does
not match any of the domains handled by the recipient of the request.
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405 Method Not Allowed
The method specified in the Request-Line is understood, but not allowed for the address
identified by the Request-URI.
The response MUST include an Allow header field containing a list of valid methods for
the indicated address.
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406 Not Acceptable
The resource identified by the request is only capable of generating response entities
that have content characteristics not acceptable according to the Accept header field
sent in the request.
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407 Proxy Authentication Required
This code is similar to 401 (Unauthorized), but indicates that the client MUST first
authenticate itself with the proxy.
This status code can be used for applications where access to the communication
channel (for example, a telephony gateway) rather than the callee requires
authentication.
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408 Request Timeout
The server could not produce a response within a suitable amount of time, for example,
if it could not determine the location of the user in time. The client MAY repeat the
request without modifications at any later time.
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410 Gone
The requested resource is no longer available at the server and no forwarding address is
known. This condition is expected to be considered permanent. If the server does not
know, or has no facility to determine, whether or not the condition is permanent, the
status code 404 (Not Found) SHOULD be used instead.
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413 Request Entity Too Large
The server is refusing to process a request because the request entity-body is larger
than the server is willing or able to process. The server MAY close the connection to
prevent the client from continuing the request.
If the condition is temporary, the server SHOULD include a Retry-After header field to
indicate that it is temporary and after what time the client MAY try again.
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414 Request-URI Too Long
The server is refusing to service the request because the Request-URI is longer than the
server is willing to interpret.
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415 Unsupported Media Type
The server is refusing to service the request because the message body of the request
is in a format not supported by the server for the requested method. The server MUST
return a list of acceptable formats using the Accept, Accept-Encoding, or Accept Language header field, depending on the specific problem with the content.
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416 Unsupported URI Scheme
The server cannot process the request because the scheme of the URI in the RequestURI is unknown to the server.
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420 Bad Extension
The server did not understand the protocol extension specified in a Proxy-Require
(Section 20.29) or Require (Section 20.32) header field. The server MUST include a list
of the unsupported extensions in an Unsupported header field in the response.
21.4.16 421 Extension Required
The UAS needs a particular extension to process the request, but this extension is not
listed in a Supported header field in the request. Responses with this status code MUST
contain a Require header field listing the required extensions.
A UAS SHOULD NOT use this response unless it truly cannot provide any
useful service to the client. Instead, if a desirable extension is
not listed in the Supported header field, servers SHOULD process the
request using baseline SIP capabilities and any extensions supported
by the client.
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423 Interval Too Brief
The server is rejecting the request because the expiration time of the resource refreshed
by the request is too short. This response can be used by a registrar to reject a
registration whose Contact header field expiration time was too small.
21.4.18 480 Temporarily Unavailable
The callee’s end system was contacted successfully but the callee is currently
unavailable (for example, is not logged in, logged in but in a state that precludes
communication with the callee, or has activated the “do not disturb” feature). The
response MAY indicate a better time to call in the Retry-After header field. The user
could also be available elsewhere (unbeknownst to this server). The reason phrase
SHOULD indicate a more precise cause as to why the callee is unavailable. This value
SHOULD be settable by the UA. Status 486 (Busy Here) MAY be used to more precisely
indicate a particular reason for the call failure.
This status is also returned by a redirect or proxy server that recognizes the user
identified by the Request-URI, but does not currently have a valid forwarding location for
that user.
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481 Call/Transaction Does Not Exist
This status indicates that the UAS received a request that does not match any existing
dialog or transaction.
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482 Loop Detected
The server has detected a loop.
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483 Too Many Hops
The server received a request that contains a Max-Forwards (Section 20.22) header
field with the value zero.
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484 Address Incomplete
The server received a request with a Request-URI that was incomplete.
Additional information SHOULD be provided in the reason phrase.
This status code allows overlapped dialing. With overlapped dialing, the client does not
know the length of the dialing string. It sends strings of increasing lengths, prompting the
user for more input, until it no longer receives a 484 (Address Incomplete) status
response.
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485 Ambiguous
The Request-URI was ambiguous. The response MAY contain a listing of possible
unambiguous addresses in Contact header fields. Revealing alternatives can infringe on
privacy of the user or the organization. It MUST be possible to configure a server to
respond with status 404 (Not Found) or to suppress the listing of possible choices for
ambiguous Request-URIs.
Example response to a request with the Request-URI sip:lee@example.com:
SIP/2.0 485 Ambiguous
Contact: Carol Lee <sip:carol.lee@example.com>
Contact: Ping Lee <sip:p.lee@example.com>
Contact: Lee M. Foote <sips:lee.foote@example.com>
Some email and voice mail systems provide this functionality. A status code separate
from 3xx is used since the semantics are different: for 300, it is assumed that the same
person or service will be reached by the choices provided. While an automated choice or
sequential search makes sense for a 3xx response, user intervention is required for a
485 (Ambiguous) response.
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486 Busy Here
The callee’s end system was contacted successfully, but the callee is currently not
willing or able to take additional calls at this end system. The response MAY indicate a
better time to call in the Retry-After header field. The user could also be available
elsewhere, such as through a voice mail service. Status 600 (Busy Everywhere)
SHOULD be used if the client knows that no other end system will be able to accept this
call.
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487 Request Terminated
The request was terminated by a BYE or CANCEL request. This response is never
returned for a CANCEL request itself.
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488 Not Acceptable Here
The response has the same meaning as 606 (Not Acceptable), but only applies to the
specific resource addressed by the Request-URI and the request may succeed
elsewhere.
A message body containing a description of media capabilities MAY be present in the
response, which is formatted according to the Accept header field in the INVITE (or
application/sdp if not present), the same as a message body in a 200 (OK) response to
an OPTIONS request.
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491 Request Pending
The request was received by a UAS that had a pending request within the same dialog.
Section 14.2 describes how such “glare” situations are resolved.
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493 Undecipherable
The request was received by a UAS that contained an encrypted MIME body for which
the recipient does not possess or will not provide an appropriate decryption key. This
response MAY have a single body containing an appropriate public key that should be
used to encrypt MIME bodies sent to this UA.
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Server Failure 5xx
5xx responses are failure responses given when a server itself has erred.
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500 Server Internal Error
The server encountered an unexpected condition that prevented it from fulfilling the
request. The client MAY display the specific error condition and MAY retry the request
after several seconds.
If the condition is temporary, the server MAY indicate when the client may retry the
request using the Retry-After header field.
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501 Not Implemented
The server does not support the functionality required to fulfill the request. This is the
appropriate response when a UAS does not recognize the request method and is not
capable of supporting it for any user. (Proxies forward all requests regardless of
method.)
Note that a 405 (Method Not Allowed) is sent when the server recognizes the request
method, but that method is not allowed or supported.
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502 Bad Gateway
The server, while acting as a gateway or proxy, received an invalid response from the
downstream server it accessed in attempting to fulfill the request.
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503 Service Unavailable
The server is temporarily unable to process the request due to a temporary overloading
or maintenance of the server. The server MAY indicate when the client should retry the
request in a Retry-After header field. If no Retry-After is given, the client MUST act as if
it had received a 500 (Server Internal Error) response.
A client (proxy or UAC) receiving a 503 (Service Unavailable) SHOULD attempt to
forward the request to an alternate server. It SHOULD NOT forward any other requests
to that server for the duration specified in the Retry-After header field, if present.
Servers MAY refuse the connection or drop the request instead of responding with 503
(Service Unavailable).
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504 Server Time-out
The server did not receive a timely response from an external server it accessed in
attempting to process the request. 408 (Request Timeout) should be used instead if
there was no response within the period specified in the Expires header field from the
upstream server.
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505 Version Not Supported
The server does not support, or refuses to support, the SIP protocol version that was
used in the request. The server is indicating that it is unable or unwilling to complete the
request using the same major version as the client, other than with this error message.
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513 Message Too Large
The server was unable to process the request since the message length exceeded its
capabilities.
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Global Failures 6xx
6xx responses indicate that a server has definitive information about a particular user,
not just the particular instance indicated in the Request-URI.
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600 Busy Everywhere
The callee’s end system was contacted successfully but the callee is busy and does not
wish to take the call at this time. The response MAY indicate a better time to call in the
Retry-After header field. If the callee does not wish to reveal the reason for declining the
call, the callee uses status code 603 (Decline) instead. This status response is returned
only if the client knows that no other end point (such as a voice mail system) will answer
the request. Otherwise, 486 (Busy Here) should be returned.
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603 Decline
The callee’s machine was successfully contacted but the user explicitly does not wish to
or cannot participate. The response MAY indicate a better time to call in the Retry-After
header field. This status response is returned only if the client knows that no other end
point will answer the request.
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604 Does Not Exist Anywhere
The server has authoritative information that the user indicated in the Request-URI does
not exist anywhere.
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606 Not Acceptable
The user’s agent was contacted successfully but some aspects of the session
description such as the requested media, bandwidth, or addressing style were not
acceptable.
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A 606 (Not Acceptable) response means that the user wishes to communicate, but
cannot adequately support the session described. The 606 (Not Acceptable) response
MAY contain a list of reasons in a Warning header field describing why the session
described cannot be supported.