I don't have T-Mobile, but I recently purchased the '07 Sedan and had the same issue... I have a Treo 700WX Cell Phone and it works okay, but it's not the best quality and it doesn't always pair with the cars bluetooth system... I'm going to trade my phone in for a Treo 700P as this is a preferred phone with Infiniti...
Go to
http://www.infiniti.com/bluetooth to make sure the phone you have is appropriate for your car.
Infiniti does not endorse or guarantee that all phones will pair properly, but they at least provide you with a list of phones that they recommend will work best....
Arizonaferrell