FreeSBC can forward SIP Subscribe, Notify and Publish messages between users and registrars (PBX). Forwarding of these messages follow the same rules as forwarding of SIP registration.
SIP Subscribe overview
The SIP SUBSCRIBE method allows users to subscribe to various services, and get notified of events related to these services. Typical services (packages) that phones will register to are:
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Voice mail
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Presence
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Conference
Each user’s device can register to various packages, then receive notifications for each of these package (independently).
Please check following link for more details; https://docs.telcobridges.com/tbwiki/SIP_subscribe_notify_publish_forwarding