1:5060;branch=z9hG4bK7154.1 Via: SIP/2.0/TCP 101.102.103.104:5060;branch=z9hG4bKa5ff4d3.1 Via: SIP/2.0/UDP 168.4.3.1:5060;branch=z9hG4bK676746 To: k<sip:explorer@geographic.org>;tag=a5642 From: <sip:explorer@geographic.org>;tag=341323 Call-ID: 67483010384@168.4.3.1 CSeq: 1 REGISTER Content-Length: 0 5.4.27 484 Address Incomplete This response indicates that the Request-URI address is not complete. This could be used in an overlap dialing scenario in PSTN interworking where digits are collected and sent until the complete telephone number is assembled by a gateway and routed [8]. Note that the follow-up INVITE requests may use the same Call-ID as the original request. An example of overlap dialing is shown in Figure 5.4. 5.4.28 485 Ambiguous This request indicates that the Request-URI was ambiguous and must be clarified in order to be processed. This occurs if the username matches a number of registrations. If the possible matching choices are returned in Contact header fields, then this response is similar to the 300 Multiple Choices response. They are slightly different, however, since the 3xx response returns equivalent choices for the same user, but the 4xx response returns alternatives that can be different users. The 3xx response can be processed without human SIP Response Messages Caller Gateway Telephone switch INVITE p:31455512@gateway.com 484 Address incomplete INVITE sip:314555123@gateway.com 484 Address incomplete INVITE sip:3145551234@gateway.com 100 Trying Figure 5.4 Overlap dialing to the PSTN with SIP. intervention, but this 4xx response requires a choice by the caller, which is why it is classified as a client error class response. A server configured to return this response must take user registration privacy into consideration; otherwise a vague or general Request-URI could be used by a rogue user agent to try to dis- cover sip or sips URIs of registered users. 5.4.29 486 Busy Here This response is used to indicate that the user agent cannot accept the call at this location. This is different, h