Notify weird behavior

I am confused by the behavior of asterisk with the Subscribe and Notify behavior.

With my Mediatrix unit, I Register (sip register that is) my unit to my asterisk server and then I receive a Notify message (no Subscribe sent) with a content-type : application/simple-message-summary. This scenario works well and my Mediatrix unit support the message-summary.

But I am confused by this scenario :
I register my Unit to another SIP server and send a Subscribe to my Asterisk (I only want the voicemail so I set this for the MWI). My unit send a Subscribe with Event : message-summary. I believe this should tell Asterisk what content-type to use in his Notify BUT when I received the Notify, the content-type is application/xpdif-xml which my unit doesn’t support.

I have tried to set in my sip.conf the line :
But it still send me an xpdif-xml format notify.

Is there anyone who can tell me why Asterisk is behaving that way and/or tell me how to force the content-type of the Notify ?

Thank you very much