MCI Communications Corp. and American Telephone & Telegraph Co. are scheduled to take their long-distance battle out of the marketplace and back into the federal courts tomorrow, with MCI hoping to win the biggest damage award in U.S. corporate history.

MCI, as part of its 10-year antitrust case against AT&T, is seeking $5.8 billion in damages. If it gets what it's asking for, the amount automatically will be trebled to $17.4 billion.

Five years ago, MCI was awarded $600 million in damages after a U.S. District Court jury found that AT&T had tried to stifle the then-fledgling telephone rival. MCI charged that AT&T had tried to monopolize the long-distance telephone market between 1969 and 1975 by charging prices below cost, delaying contract negotiations with its competitors and refusing to give MCI essential telephone hookups to the Bell System so calls could be completed.

AT&T defended its actions by claiming that it not only was following Federal Communications Commission rules but also was protecting the public interest.

The jury found AT&T guilty of 10 of 15 allegations and granted the $600 million award, $300 million less than MCI originally sought. The $600 million award automatically was trebled to $1.8 billion. The Clayton Antitrust Act of 1914 provides for trebling of damages as an additional deterrent to companies that violate antitrust laws.

But the 7th U.S. Circuit Court of Appeals overturned the award in January 1983 and ordered a new trial to set damages. The appeals court rejected the jury's finding that AT&T had engaged in predatory pricing against MCI, but essentially upheld the finding that AT&T had illegally refused to interconnect MCI to its local exchanges. The case was sent back to the lower court for a new trial on damages based only on the interconnection charges.

The Supreme Court refused to hear the case on appeal in October 1983, and both companies claimed victory.

In Chicago last week, U.S. District Court Judge John F. Grady chose the 12-member jury to try the damages portion of the suit. The jury consists of seven women and five men from the Chicago suburbs. Opening arguments are scheduled to begin tomorrow.

The jury includes an engineer, a heating system designer, an accountant from International Harvester Co., a purchasing agent from Caterpillar Tractor Co., an aircraft mechanic from Flying Tiger Line Inc., a chemist, a loan officer from a nearby bank, a secretary, a housewife, a clerical employe and a college employe.

MCI says it is confident that it will be awarded the $5.8 billion it is now requesting. "If I didn't think we were entitled to get it, I wouldn't be asking for it," said John R. Worthington, MCI's general counsel.

At the trial, MCI will present an economic study of what it claims it would have earned in revenue and profits if it hadn't been for AT&T's antitrust violations.

"We have the actual MCI company, we know what it did. From that we will extrapolate what MCI would have done if it were not for the delay occasioned by AT&T's antitrust violations," Worthington said. "Then you compare the MCI that would have been, and the difference is the $5.8 billion."

In the last trial, MCI had developed figures for damages only through 1979, said Worthington, but the study takes into account MCI's financial picture through 1984.

The original projections "turned out to be extraordinarily conservative when looked at against what MCI accomplished from 1980 to 1984. . . . In the initial trial, what we thought would happen was a very, very low-ball figure," he said.

Essentially, AT&T "cut MCI's start" by causing MCI to start competing later and more slowly in the long-distance market, Worthington argues. He said the company also received lower-quality connections to the local telephone network than AT&T enjoyed.

"We would have had many more customers and our profits would have been much higher," said Worthington. As it is, MCI's overall revenue grew almost 1,000 percent to $1.96 billion between 1980 and 1984, and therefore, its revenue and profits could have been much larger without AT&T's interference, said Howard Crane, senior vice president for MCI.

"That's outrageous," Blair White, chief counsel to AT&T and a lawyer with the Chicago law firm Sidley & Austin, said of the new MCI damage request.

At the time of the last trial, MCI had not yet started to provide residential service on a mass-market scale and based its request solely on lost profits in its business private-line services, White said. But now, he added, MCI is claiming that AT&T hurt its entire business.

White said that, in essence, MCI is claiming that damages to the company's finances do not show up until 1985. The damages, he added, are "based on certain margin assumptions which, from our standpoint, are wholly unrealistic in the face of what is going on in the real world."

White said that the long-distance market is vastly different from what MCI assumes it would have been, given the steep rise in prices long-distance companies must pay to local phone companies for connection to the local phone networks.

These connections, called "equal access" lines, give consumers the ability to reach any long-distance company without dialing lengthy access codes. AT&T pays the full price for such connections, but its competitors are beginning to pay the full cost as they receive connections to the local network of the same quality that AT&T enjoys. That has made profit margins much thinner this year for AT&T's competitors.

AT&T will present a study at the trial analyzing damages that can be traced to the remaining liability findings, White said. "MCI lost all the important charges; our estimate of the amount of damages before taxes is something under $10 million."

Analysts, however, think MCI is likely to walk away with more than that -- though not as much as the company has requested. "I think MCI will get a significant amount of money -- perhaps between $500 million and $2 billion to $3 billion," said James Mason McCabe, an analyst with Prudential Bache Securities Inc. "A billion dollars would certainly help them -- that's a year's capital expenses."

The jury's decision on the amount of damages may be tempered by the change in the telecommunications world's climate since the breakup of AT&T, said Philip Verveer, a partner and lawyer with the law firm of Willkie Farr & Gallagher. Verveer initiated a separate Justice Department antitrust case against AT&T that eventually led to the breakup of the Bell System.

"I think MCI expects it will come away with substantial damages, and I think that's right -- but the extent of the damages may be tempered substantially by a very different-looking AT&T today," Verveer said.

During the last trial, AT&T "probably looked like a much deeper pocket in the payment of damages and like a company much more formidable and fearsome-looking in terms of its competitive activities," he said. "It doesn't look that way anymore."

AT&T has had its own share of financial troubles after spinning off its seven regional telephone companies and has encountered a rough new long-distance market as well as tough competition in its telecommunications equipment and computer businesses.

One antitrust lawyer who asked not to be identified said that in the last trial, "the case for AT&T's liability was very, very good, and the case for damages was very flaky."

The weakness in the damages argument, said Theodore Whitehouse, an associate at Willkie Farr & Gallagher who worked on the Justice Department case against AT&T, is that MCI bases damages on projections about a service rather than a product.

"It is easy to figure out how much damages are if I sell widgets and you don't follow through on a purchase contract," he said.

"It is highly speculative to figure out damages if I claim my ability to sell a service has been impaired by your conduct," he added.

Nevertheless, any little bit of a damage award will help MCI. Any long-distance companies "that want to be survivors need as much cash as they can lay their hands on," Verveer said.

MCI's profits plummeted last year to $59.2 million from $155.7 million in 1983 because of increased competition and rising costs for connections to local phone networks. But Jennifer Proga, an MCI analyst with Lehman Bros., said the company is in good shape.

MCI spent more than $1 billion to expand its network last year, and still had $865 million in cash at the end of the year, she said.

"They are in fine shape through 1986 and plan to spend $900 million in 1985 and $850 million in 1986 to build their network," she aid. "They don't need the money over the next two years to complete their capital expenditure program."

A large award would mean the company would not need new loans until 1990, she said.

"It would mean we wouldn't have to go to the market and raise new capital funds for a while," said MCI's Worthington. "It would allow us to expand our business on a very rapid scale, go to places we haven't built and increase capacity between places where we already are." The money also would help the company diversify into other areas of business telecommunications, he said.

MCI already has settled out of court a portion of the same suit, which it filed against all seven regional telephone companies as well as their former parent, AT&T. MCI said its settlement with the Denver-based US West Inc. regional telephone company, which owns three local phone companies, included cash payments of an undisclosed amount and "business relationships" that will benefit MCI.

Because the seven regional telephone companies are named as defendants in the antitrust suit, any damages would have to be split among AT&T and its former operating companies. FCC officials said the stockholders in the companies, rather than the ratepayers, would have to assume the damages.

"They would have a very difficult case to make to argue liability for violation of antitrust laws to be something they can properly recover from the ordinary subscriber," said the official who asked not to be identified.