I'm trying to understand what kind of standard protocols can be used for "Raise hand" function in SIP VoIP calls. I see this function is implemented in many products, but I can't find any standard protocol definition for this.
Consider we have 3 entities: Host (call leader, Alice), Conference Server, Endpoint (regular participant, Bob).
Scenario: Bob presses "Raise Hand" button. Alice looks on a list of users in conference, and sees that Bob raised a hand.
What I was able to find so far:
Alice: can send SIP SUBSCRIBE to server to get NOTIFY messages about users in a conference (RFC4575 https://www.rfc-editor.org/rfc/rfc4575)
Bob: it can simply send DTMF code like *9 to conference server to raise a hand
Conference SIP Server: how it will notify Alice that Bob has raised a hand? Should it be also a NOTIFY message? In which format?
Alice: answered a question and wants to "lower Bob's hand". How it can be achieved in standard way?