5.3.1. Appearance Numbers and Call Context

There are cases where two separate dialogs at a UA are not mixed but share the same 'context'. That is, they relate to each other and should not be treated the same as any other two dialogs within the group. One example of this is a 'consultation call' where a user puts an existing dialog on hold, then calls another user, before switching back to the original dialog. Another case, described below, occurs during transfer operations, where for a transient period, a UA is involved in dialogs with two other UAs, but the dialogs are related, and should not be treated as independent dialogs. These cases are best handled by not assigning an appearance number to a newly created dialog when it shares a context with an existing dialog. But if the preexisting dialog is terminated, its appearance number should be reassigned to the newly created dialog.

A UA that wants to place a call but does not have an appearance number assigned sends a PUBLISH before sending the INVITE. The PUBLISH does not have an 'appearance' element present, but it does have the 'shared' Event header field parameter present. If the Appearance Agent policy does not allow calls without an assigned appearance number, a 400 (Bad Request) response is sent by the Appearance Agent and the UA will republish either selecting/seizing an appearance number or send the INVITE without publishing, in which case the Appearance Agent will assign one.

Note that if an Appearance Agent rejects calls without an appearance number, certain operations such as consultation calls, transfer, and music on hold may be negatively impacted.