SIP SUBSCRIBE/NOTIFY

SIP methods for event notification — SUBSCRIBE requests ongoing updates on a resource (presence status, call state, message waiting), and NOTIFY delivers those state changes to the subscriber. A SUBSCRIBE creates a subscription with an expiry; the subscriber must re-SUBSCRIBE before expiry to keep it active. NOTIFY is sent immediately after SUBSCRIBE with the current state and then again whenever the state changes. This mechanism is used by BLF (Busy Lamp Field) indicators and presence systems.