If you are going to use a wireless phone in your car, some kind of Bluetooth device is an absolute must. Not only is hands-free required by law nearly everywhere, it is simply safer, period. Frankly its also considerably more convenient.
If you are in the market for a new vehicle, Bluetooth will be standard equipment. If you are in the pre-owned market, be sure to ask or to look for the Bluetooth logo (right),
Available aftermarket devices range from as simple as an earpiece through add-on devices that are essentially advanced speaker phones and on through built in entertainment centers that automatically can pick up and make calls through voice commands.