How to set MWI LED on ISDN phone?

Anyone knows how to control the Message Waiting Indicator of an ISDN phone? Is there a standard way to do this, or is it device dependent? Is there support for such control in *?
I have read about the ‘hint’ priority and the ‘Devstate’ call, but this seems to apply to snom phones only.

I would like to set the LED to indicate voicemail messages waiting. I am using the zaphfc driver controlling an hfc card in NT mode to connect to an Ascom Eurit 22 ISDN phone.