It got really nasty two years ago when Joseph P. Nacchio left AT&T. After criticizing then-CEO Robert E. Allen's business strategy, the phone company's head of consumer operations found himself muzzled in high-level meetings and desperate to make a getaway. ''You don't expect a callous attitude as if you're in grade school, where you can comment only in a limited way,'' the hard-charging New York native recalls. Adding injury to insult: AT&T tried to deny Nacchio millions of dollars in benefits when he left.

Now, Nacchio is exacting his revenge. Allen is out at AT&T. The company is struggling to find a growth strategy. And Nacchio, 48, is getting rich running one of the hottest upstarts in telecom, Qwest Communications International. He joined the company in January, 1997, after meeting Qwest's owner, Philip F. Anschutz, a reclusive dealmaker who had amassed billions in oil and railroads and now was aiming to build a broadband, high-speed communications network.

Nacchio is off to a galloping start. Virtually all of Qwest's 26,208-km network connecting 125 U.S. cities will be completed by the end of this year. The company has launched 7.5 cents-a-minute long-distance service for consumers and is targeting businesses as well.

Nacchio has got plenty of room to take on more customers. Each of Qwest's 48 fibers can handle 500,000 simultaneous telephone conversations--about 12 times more than a traditional phone network is capable of. And even though the company is only using two fibers, Nacchio reckons that's enough to handle all 400 billion minutes of current U.S. telecom traffic.

All this has made Nacchio the poster boy for the ''next-generation telcos'' and Denver-based Qwest a rising star on Wall Street. Analysts project revenues will soar nearly 50% this year, to almost $1 billion, but construction costs will cause a loss of some $50 million after net income of $14.5 million in 1997. Qwest's stock has nearly tripled, to $40, since its initial public offering last June. And on Mar. 9, Qwest agreed to acquire LCI International Inc., a company with twice the revenues of Qwest--for $4.4 billion in stock. ''I'm the most surprised guy of anyone at how well we've done,'' Nacchio says. Sitting on stock options valued at some $200 million, revenge has seldom been sweeter.

By Steven V. Brull in Los Angeles