Lawyers for American Telephone & Telegraph Co. and MCI Communications Corp. have a small bet. When their bitter $2.7 billion lawsuit ends, the loser of the wage will stage a party for the winner. Whether anyone on the trial teams attends is another story.

"I don't want to give the impression that we'll go out and have a big party when this is over," said Robert Hanley, MCI's lead attorney. Maybe some of the younger attorneys will got to a party. I won't. I don't like the way they (AT&T) are operating."

For the story of MCI v. AT&T is something more than a tale of high-priced legal talent politely arguing in a courtroom. It is the story of the bitterness, humor and tension that surfaces in a courtoom where the weight of an already extraordinarily expensive lawsuit hangs in the balance.

In a tense Chicago courtoom since Feb. 4 (with a five-week break in the middle before AT&T opened its defense), some of the nation's highest-priced legal talent has been arguing the merits of MCI's $2.7 billion antitrust suit against AT&T, which is by many standards the nation's largest company.

The suit, which an AT&T lawyer calls as "complex a case as I have ever seen," may in fact be that complicated. It revolves around communications lingo, as the attorneys bat around such terms as "CCSA," "interconnections" and "specialized common carriers." They repeat to a frequently befuddled-looking jury a litany of Federal Communications Commission decisions, like school boys arguing about their heroes' batting averages.

Thousands of pages of exhibits have surfaced in the case as dozens of lawyers, paralegals and administrative assistants scurry about the courtroom pushing papers in front of their superiors. AT&T has about 60 persons working on the case, a spokesman said. The staff for MCI is only slightly smaller.

All this may not seem to unusual to afficionados of regulatory proceedings, particularly those involving communications law.

But what is startling to observers is the tone of the proceedings. The trial pits two of the nation's most prestigious law firms, Chicago's Sidley & Austin representing AT&T and Jenner & Block representing MCI.

What is just as unusual is how acutely the fates of both Sidley & Austin and AT&T are tied. The top lawyer in the Chicago firm, Howard Trienens, wears two hats. He not only runs a staff of about 250 lawyers for Sidley, but also is AT&T's vice president and general counsel. Further, Sidley incorporated AT&T's Western Electric Subsidiary.

When the case finally concludes in a few years -- after a counter suit, an inevitable series of appeals, and second MCI lawsuit are over -- there may well be a party. But if that bash is like the trial itself, the get-together may turn into a brawl.

The lawyers call each other liars, publicly and privately.

U.S. District Judge John F. Grady has had surprisingly harsh things to say about the FCC's landmark 1971 decision, which opened the telecommunications field to new competition, and has frequently criticized the behavior of the press in covering this case and others.

"Sure, there's animosity here," said Robert Hanley, a member of Jenner & Block and the lead MCI attorney. "I'm personally offended by the abuse of discovery by AT&T, among other things. They took hundreds of depositions they didn't need. I'm upset about the way they're trying the case. I think they're giving us the same kind of treatment AT&T gave MCI."

George Saunders, AT&T's attorney in the MCI case and its lead attorney in the Justice Department's case against the Bell System, has a similar view of his courtroom opposition. "MCI uses AT&T as a whipping boy to get attention, favors from the government and publicity," Saunders said. "And they do it quite effectively."

But MCI Chairman William McGowan points out that MCI may be AT&T's private customer, paying the Bell System close to $4 million each month for use of its lines. That fact creates a unique situation in the industry, typified by this case. Unlike other industries, MCI could not have survived without the hookups with AT&T.

"I don't hate them at all," McGowan said. "I'm running a business. They tried to put me out of business." McGowan, however, has even stronger feelings about Saunders. "He's been making wild statements and he'll eat them when the trial is over," McGowan said this week.

Grady himself, is no one to mince words. In December, when the six-year-old case was scheduled to go to trial, Grady postponed it for two months after newspaper stories were published quoting from a public order written by Grady, who had called AT&T's trial plans "grandiose."

Although it probably will not take AT&T more than five weeks for its defense, the corporation's attorneys had originally proposed an 18-month presentation. Later that estimate was cut to eight months, persuading Grady to write the lines that resulted in those controversial stories.

"A reading of the defendant's list and the proposed subject matter of its testimony persaudes me that defendants have been thinking more in terms of a theatrical production than the trial of a lawsuit," Grady wrote last fall.

Just as stinging were his comments after another newspaper story quoted attorneys in the case. After Saunders complained in open court about a quote in that story and suggested that he might write to the editors of the newspaper, Grady said such a tactic was hopeless.

"Once you give it to the newspapers, it is a no-win proposition," Grady said. "It is just a bottomless pit."

His comments about the FCC decision, a major part of the case, were just as stinging. He has called the decision "unbelievable," and at another point said that if FCC staff members wee actually "experts, they would have written a decision that was comprehensible . . . one of the worst examples of legal draftsmanship I have ever seen."

"It is just inconceivable to me that they would write a decision of a hundred and some pages and never once say what it was they are talking about," Grady said at another time the other day.

But the very decision that Grady has often chided during the trial is the drux of the case. "The issue is this," Grady said: "To what extent did the FCC change the regulations . . . when it issued the common-carrier decision."

That decision, in essence, opened the door for companies to compete with AT&T in providing long-distance telephone service. It is a decision that MCI says immediately entitled the Washington-based firm to access to AT&T's specialized telephone services.

But MCI contends that in the meantime they lost hundreds of millions of dollars of business. Since then the firm has grown dramatically; it expects to report 1979 revenues of more than $100 million. Not a tiny firm, by most standards, but one that will have little to worry about if it wins it suit.