call could be returned with a new INVITE and the Call-ID from the missed INVITE copied into the In-Reply-To header field. This allows the UAS to determine that this is not an unsolicited call, which could be used to override call screening logic, for example. Examples of this header field are as follows: In-Reply-To: a8-43-73-ff-43@company.com In-Reply-To 12934375@persistance.org, 12934376@persistance.org 6.2.10 Join The Join header field [8] is used in an INVITE to request that the dialog (ses- sion) be joined with an existing dialog (session). The parameters of the Join header field identify the dialog by the Call-ID, To tag, and From tag in a similar way to the Replaces header field. If the Join header field references a point-to-point dialog between two user agents, the Join header field is effectively a request to turn the call into a conference call. If the dialog is already part of a conference, the Join header field is a request to be added into the conference. An example call flow is shown in Figure 6.2 in which a two-way call is turned into a conference call. SIP: Understanding the Session Initiation Protocol Alice Carol Media session INVITE (Join) 180 Ringing INVITE 200 OK Conference media session 200 OK Conference media session Figure 6.2 Use of Join to create a conference call. If the dialog referenced in the Join header field does not exist, a 481 Call/Dialog Does Not Exist response is returned. A UA supporting Join should indicate this in all requests with a Supported: join header field. In the following example, the dialog: To: <sip:moe@example.org>;tag=42312 From: <sip: larry@server.org>; g=3443212 Call-ID: 243134123234 would match the Join header field: Join: 243134123234; to-tag=42312; from-tag=3443212 6.2.11 Priority The Priority header field is used by a UAC to set the urgency of a request. Defined values are non-urgent, normal, urgent, and emergency. This header field could be used to override screening or by servers in load- shedding mechanisms. Because this header field is set by the user 