FreeSWITCH, contrary to popular belief, does support Subscriptions and MWI.
In order to enable Subscriptions, add the following line to the profile settings.
<param name="manage-presence" value="true"/>
Notifications can be sent to the user in all the same ways that other events can be. (i.e. js, socket, etc)
mwi.js (generic MWI template js)
More to come as I get a more complete working deployment.
MWI if you are using FreeSWITCH as a voicemail server, but registering to OpenSIPS etc
All you have to do is setup OpenSIPS to relay the SUBSCRIBE messages to FreeSWITCH and it just works.