Cell phones can be used to make calls in cars standing with their engines switched off as well as by passengers. For reasons of safety on the road, however, laws in Germany prohibit drivers from making phone calls without hands-free equipment or a headset while they are driving. Please note also that any diversion of the driver increases the risk of accidents.
In the context of safe cell or smart phone usage, it is important to know that the exposure limits are also complied with when you make a mobile call in your car. The car's metal bodywork shields connection quality slightly. However, this does not result in any significant increase in the electromagnetic fields emitted by the cell phone. The electromagnetic fields generated during car-based phone calls can be reduced by using an external antenna.