I know it works, cos the salesman showed me it in my exact car before I collected it, he used voice control to enter sat nav destination.
Since I picked the car up, I have paired my iPhone, and the Voice button on steering wheel, when held in for a second, activates Siri on the phone, and the little display on the dash says "External Device". Now, Siri is no use for entering commands to the BMW iDrive system, so I went through all the settings to see if there's one to make the button activate the BMW voice system instead of Siri. No such setting.
So I thought I'd simply turn Bluetooth off on my phone to stop it connecting, just to see if it works. Nope. Now the voice button on steering wheel does nothing at all. Doesn't matter if I press it or hold it in, nothing. I even tried pressing/holding in and then speaking, just in case there was no obvious display/audio tone to tell you it's expecting a command, still nothing.