did you make in progress with this… i have been working with chan_sip to get more features out of my avaya. phones ( Avaya 96x1 extended Features )
i ran in to something similar only to run in to an issue with subscribes needing to be reauthorized when the phone was not expecting too.
if you still having issues i should be able to come up with the correct code. or you can use the post above for a starting point to update chan_sip.
How did you configure freepbx to send the MWI ?