I recently purchased a 2005 c230 kompressor. The car has the phone kit installed (and has the nav system). I want to get a phone to use with it. Dealer says i can ONLY use the MERCEDES Motorola V710. Is there anything esle that is compatible? Can I not buy a v710 from AT&T? Does it HAVE To be the Mercedes version that they want to charge me $575 for !?!?!?