When a user attempts to sign in with a Jabber Video for Cisco TelePresence client, which SIP message is the first that is sent?

SIP INFO is an in-dialog message used to carry application-level information mid-call and is never the first message sent during a client sign-in sequence.

SIP NOTIFY is sent by the server to inform a subscriber of state changes and is not initiated by the client as the first sign-in message.

While REGISTER is eventually sent during the sign-in flow, SUBSCRIBE precedes it in the Jabber Video sign-in sequence as the client first subscribes to services.

Jabber Video uses the SIP SUBSCRIBE method as the initial message during sign-in to subscribe to its own presence state and service availability on the VCS or CUCM, which is part of the Cisco-specific SIP sign-in flow before REGISTER is processed.