I remember seeing lots of posts about various cars' bluetooth features working or not working with the Treo 650. I can't speak to other models, but I remember that people were having trouble with the Sprint Treo 650 and the Toyota Prius. I've been able to successsfully pair my unlocked GSM Treo 650 with my Toyota Prius and both make and receive phone calls through the car. The only caveat is that for some reason it doesn't work if I use the phone to dial - I have to dial through the car. I can also transfer phone numbers to the car, though I have to sometimes try the transfer twice.