eption of 100 Trying must con- tain a Contact URI and echo all Record-Route headers received in the request. However, a RFC 2543 implementation will not do this, as it was not mandated in that document. 5.1.1 100 Trying This special case response is only a hop-by-hop request. It is never forwarded and may not contain a message body. A forking proxy must send a 100 Try- ing response, since the extended search being performed may take a significant amount of time. This response can be generated by either a proxy server or a user agent. It only indicates that some kind of action is being taken to process the call-it does not indicate that the user has been located. A 100 Trying response typically does not contain a To tag. 5.1.2 180 Ringing This response is used to indicate that the INVITE has been received by the user agent and that alerting is taken place. This response is important in interwork- ing with telephony protocols, and it is typically mapped to messages such as an ISDN Progress or ISUP Address Complete Message (ACM) [2]. When the user agent answers immediately, a 200 OK is sent without a 180 Ringing; this scenario is called the "fast answer" case in telephony. A message body in this response could be used to carry QoS or security information, or to convey ring tone or animations from the UAS to the UAC. A UA normally generates its own ringback tone or remote ringing indica- tion, unless a Alert-Info header field (see Section 6.1.1) is present. 5.1.3 181 Call Is Being Forwarded This response is used to indicate that the call has been handed off to another end-point. This response is sent when this information may be of use to the caller. Also, because a forwarding operation may take longer for the call to be answered, this response gives a status for the caller. 5.1.4 182 Call Queued This response is used to indicate that the INVITE has been received, and will be processed in a queue. The reason phrase can be used to indicate the estimated wait time or the number of callers in line, as shown in Figure 5.