Bluetooth technology originated in 1998, but it went through many years of uncertainty before it caught on and became universal.

Today, many devices use this technology, but most recognizable is the cellular phone. Whether required by law or not, everyone understands the safety of hands-free phone use in the car.

Compared to earlier years, when proprietary hands-free equipment had to be installed to match each cellular phone, Bluetooth now brings all devices together to work harmoniously, without constant hardware changes. A one time installation of a Bluetooth hands-free system is required, then phones can be "paired" and "unpaired" at will.
Almost all new high end cars are now outfitted with a factory Bluetooth system. But for those that are not:

Many earlier Audi, BMW and Porsche vehicles that have navigation, can be fit with a system that emulates the factory Bluetooth system or replaces the earlier hands-free phone cradle. Depending on the vehicle; radio, and steering wheel controls can be used while displaying information on the screen. Compatible units cost $1,000 installed.

For any vehicle, I recommend the Motorola IHF1700. There are many systems on the market, most try to lure customers with displays, buttons and unnecessary features. I find customers prefer a simple, clean installation that is still packed with features. The only part visible on the dash is a stylish, back-lit control. The system utilizes voice activation, either to bring up a stored location or even dial a number by voice. Incoming calls will announce the caller if their number is in memory. The system can handle up to 4 phones being paired to it, changing phones, is also a voice command. Finally, it is solid and reliable Motorola technology.

Cost is $800 installed.