S20 and Trunk's voicemail MWI

I recently got the S20 and I am facing a problem. At the moment I use Provider's voicemail. Before getting the S20 I was using a gigaset dect voip phone and every time I had a voicemail the red LED on the gigaset would be flashing. 

Now that I use the S20 the gigaset dect phone is an extention and when I get a voice mail (Provider's voicemail) I don't get the flashing red light on my gigaset (MWI)

My question is how do I get S20 to get the MWI from provider and forward it to my gigaset extention