(FORTUNE Small Business) – Next to an Abacus, perhaps, there is no piece of office equipment that shrieks "old economy" more loudly than a fax machine. Yet J2 Global Communications has built its business around providing fax services--and earned the fourth spot on this year's FSB 100 ranking of the fastest-growing small public companies. "Everyone thinks fax is going the way of the buggy whip," says Scott Jarus, 49, president of J2 Global, which is based in Hollywood, Calif. "Well, just try buying a house without using it. Fax is still ubiquitous."

For $12.95 a month, a customer leases a telephone number from J2 Global, and the faxes sent to that number are automatically translated into e-mail attachments. The company's core clients are business travelers, who regard the service as a convenience on a par with rolling luggage. And purchasers are remarkably loyal. The annual turnover is less than 3%.

How much demand could possibly exist for such an Ice Age--era offering ? Last year J2 Global's revenues soared to $106 million, more than triple what they were three years earlier. Several analysts predict that revenues will reach $148 million this year, with an operating profit margin of around 40%. During the first six months of 2005, revenues rose 38%, to $67.1 million. Net climbed 58%, to $21.9 million. The company's stock has gone up steadily, hitting a 52-week high of $42.16 in March and still hovering at around $40 as of early August.

J2 Global sells other services, such as voicemail to e-mail and conference calling, but fax remains its biggest moneymaker--a business model that Jarus doesn't expect will change in the foreseeable future. He's well aware that sales of fax machines in the U.S. peaked in 1997. Three years later President Clinton signed a bill that granted "electronic signatures"--unique digitally encrypted codes that could replace pen scrawls--the same legal status as old-fashioned autographs. But most industries have yet to acquire the technology needed to accept electronic contracts. Same with state courts. Federal laws such as Sarbanes-Oxley and HIPAA have only stacked up more paper.

Experts predict that the global fax market will grow 10% this year. As its name suggests, J2 Global has already followed its customers into 25 countries. To maintain its competitive edge--yes, rivals exist, including Miami-based Venali and Data on Call in San Diego--the company keeps an inventory of 12 million phone numbers, concentrated in the most popular area codes--212 in New York and 213 in Los Angeles, for example. (Another fax-based business, Castelle, which is based in Morgan Hill, Calif., and which won the 50th slot in this year's FSB 100, is in a different end of the industry, selling fax servers that allow customers to send and receive faxes on their personal computers. Says CEO Scott McDonald: "Fax isn't sexy, but it is essential.")

Four years ago J2, which was founded in 1995, could hardly have been less glamorous. In late 2000 its stock sank below $1, and Nasdaq threatened to delist it. In 2001 the company moved to a low-rent district of Los Angeles, sharing the neighborhood with hookers and panhandlers. That was the same year that investors brought in Jarus, a telecom veteran. He turned the company around partly by using a marketing gimmick: J2 lets prospects try its service free (those users also receive e-mail advertising). After a four- to six-month trial, many sign on as paying customers. The current tally: 8.4 million freeloaders, 400,000 paying customers. "I have a lot of runway ahead of me," insists Jarus, who sees a bright future in something that only looks old economy.