The SIP specification has been extended over time to support a general mechanism allowing for subscription to asynchronous events. Such events can include SIP proxy statistics changes, presence information, session changes and so on.

A user agent interested in event notification sends a SUBSCRIBE message to an SIP server. The SUBSCRIBE message establishes a dialog and is immediately followed by the server replying with 200 OK response. At this point the dialog is established. The server sends a NOTIFY request to the user every time the event to which the user subscribed changes. NOTIFY messages are sent within the dialog established by the SUBSCRIBE.

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Understanding REGISTER method

SIP is a peer-to-peer protocol where the roles client – server and exchangeable depending on who starts a session. In reality most deployments foresee a process called registration (method: REGISTER) which allows a central server (registrar) to store the location of a SIP User-Agent.

A SIP Phone is a client to the central Unified Communication Platform (registrar) – and the UC platform is a client to the SIP Server of the operator (registrar).

Once the SIP Server gets to know the location of an SIP Client, it can deliver calls and other messages to it from other Clients connected to the same Server.

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