(FORTUNE Magazine) – A cellular phone can turn a car into an office on wheels. But before buying one of these great communicators, consider: Cellular equipment is not cheap. Although the price of phones is dropping, expect to pay about $1,200 for an installed car phone and up to $3,500 for a portable model. Monthly charges vary by city. An average monthly bill for five hours of calls during normal workdays will run $150. The bill includes all completed calls whether placed or received; some systems even charge when the line is busy. Look for a full-service dealer to sell, install, and service the phone. Usually installation is included in the price of the phone. If it's not, count on paying up to $200 more. Check the manufacturer's warranty, dealer certification, and length of service provided. Thinking about a portable? Ask to try one out. Cellular systems were not designed with portables in mind, so while the phone may work fine in the car or on the golf course, it may fail indoors. The antenna is perhaps the most important determinant of how well the car phone will work. Ideally the antenna, which resembles a radio antenna with a corkscrew in the middle, should be mounted on the car roof. But most people don't want their vehicles to look like cop cars in disguise and opt for an antenna on the fender. Don't dial and drive. The cellular phone industry says that accidents are no more frequent for mobile phoners than for other drivers. But why take chances? Many phones come with free-standing speakers and shortcut programmed dialing, which allow for both eyes on the road. An additional $400 to $500 gets a voice-activated model that will respond when you tell it, ''Phone home.''