It is possible. I created a blog entry from a mailing list thread doing this sort of stuff for Asterisk behind Kamailio and voicemail MWI .
http://saevolgo.blogspot.ca/2012/07/asterisk-behind-kamailio-voicemail-mwi.html?m=1
The scripts and everything should just give you idea. In Kamailio when you get Subscribe create a file in temp folder with User in it. Take a cron job that runs a sipsak based script to lookup any files in that temp folder. Based on that username do w/e you want and create a SIP packet and send it back to Kamailio.
I hope this may get you some ideas.
I see, what i’m trying to do is actually to limit querying the DB by updating the MWI only when there is activity.
In other words to send a NOTIFY when the user receives a new message in his voicemailbox.
I am not sure if it’s possible to send the NOTIFY for example 5-30 mins after the initial subscribe.
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