I like your thinking that everything should work, however voice control (Mercedes and BMW) I believe is designed to be recalcitrant and disobedient !! You know its working correctly when the command is "Call Home" and the answer is " Navigation Off" or "Radio 2" My last Merc had problems similar to the one you describe, the Dealer could never find a fault yet if I unplugged the voice control unit and reconnected it it would work sometimes for months. The telephone function is the most useful. In the X5 I had problems but changed the unit with one from Ebay no rebellion in the last 6 months