Hands-free speakerphones for use in cars come under bluetooth for cars. When a call comes in on a smartphone (eg iPhone), the call goes into the car system if the smartphone is docked, and the call operates immediately through the car's system, and everyone in the car can listen to the conversation. This is alright for business calls and if the message can be heard by others.
However, if the call is private and to a person (the car driver), if the call accidentally goes into the car's speakerphone system or transmitted through the FM channel, it can be embarrassing for the others to listen to the personal call. In this case, the caller must alert by texting (SMS) the car driver first and then only call, so that the car's speakerphone is switched off and the call taken personally, without the others listening to what is said.
Bluetooth are available in most highly priced cars such as the Mercedes, BMW and Audi.
YouTube video on Bluetooth for Cars
Audi Telematics
SuperTooth buddy
Bluetooth for Toyota cars
