Some cars today have hands-free Bluetooth smartphone capability
built in, such that calls automatically come in over your car’s
speaker system and a built-in microphone picks up your end of
the conversation.
If you don’t have a car with that feature, you
may want to consider the Griffin SmartTalk Solar, which is a small
Bluetooth speaker in a case that attaches to your windshield with
suction cups.
It has a built-in solar panel to keep the internal battery
charged.