For this reason, various network entities within the IMS may send a provisional response.
The provisional response indicates that the network is delivering the request
to the destination, and it may even provide some form of status.
This prevents expiry of the request timer and lets the device know that the request
is being delivered. There are several provisional responses supported:
?– 100 Trying Indicates that the network is attempting to reach the destination
(sent by a proxy to prevent retransmission by the requestor).
?– 180 Ringing This is sent by the request recipient to indicate the request has
been received and the device is alerting the subscriber.
?– 181 Call is being forwarded Indicates the call is being forwarded to another
number.
?– 182 Queued Indicates the called party is not available, and the receiver of the
request is going to queue the request for later delivery.
?– 183 Session Progress This is sent by intermediary entities to indicate the session
request is being routed through the network and is being processed by the various
entities. For example, a Media Gateway Controller (MGC) receives a request
and immediately returns this response to indicate it is processing the request.
2xx Successful Responses Success responses are sent by the receiver of a request
once the receiver has agreed to establish a dialog with the requestor.
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