my iphone4s used to work no prob by pressing the home screen.
you should check your settings and make sure it works without the car using the home screen (although im sure you already did this)
next is the problem with the e90 and the type of blue tooth siri sends out. apparently its different than what calls are used for.
i know my samsung s3 s-voice doesnt work over BT e90 but the calls work perfect. Its because the calls are sent using basic bt mono i think. and siri and s-voice use some fancier bt
cant say for sure this is your problem, but it could be the case.
fix for me was installing an app that forced BT mono from s-voice