(FORTUNE Magazine) – About ten million Americans have cellular phones, and new subscribers are signing up at the rate of 220,000 a month. Car phones originated the trend just nine years ago, but the fastest-growing market segment now belongs to hand-held models of the ilk favored by communicoholic execs. Portables made their debut less than three years ago but already make up an estimated 30% of the cellular market and some 45% of new sales. Who's ringing up the revenues? Industry analyst Herschel Shosteck of Silver Spring, Maryland, says that manufacturer Motorola has more than 30% of 1992's $1.4 billion market for the equipment. McCaw Cellular Communications is the market leader among companies transmitting the calls, with a 21% share. McCaw, the Baby Bells, and others will collect an estimated $7.2 billion this year. Long-distance calls are extra.