he bribery-conspiracy trial of Teamsters Union president Roy Lee Williams and four other defendants was abruptly interrupted today when five of the jurors reported receiving intrusive phone calls about the case at their homes early this morning.

The calls were placed, apparently by the same gravelly voiced male, between 6:30 a.m. and 7:15 a.m., according to U.S. District Court Judge Prentice H. Marshall.

After day-long, closed-door questioning and consultations with lawyers in the case, Marshall announced that the jurors would be sequestered for the remainder of the trial.

Marshall also said he has been forced to postpone the proceedings until Monday because Williams "has become quite ill."

The Teamsters president, described as suffering from emphysema, has coughed heavily throughout the trial. "The prognosis is that he will be able to proceed on Monday, but the illness is acute," said Marshall, adding that Williams is under a physician's care.

"It is just one of those little quirks of fate that these two incidents coincided," Marshall said of Williams' affliction and the phone calls to the jurors.

The five jurors were not identified, and the judge told them not to discuss the incident with each other or with jurors who did not receive calls.

Twelve jurors and five alternates have been hearing the case since opening arguments began Oct. 19.

In open court, Marshall gave only a terse description of the calls, saying he did not think it appropriate to disclose what was said. He told the jury that he decided on sequestration because "I think it reasonable to assume that the person that made the calls . . . might try to make calls to you in the future."

Some of the jurors hung up quickly on the caller, and another apparently did not hear much because of background noise, the judge said. He said that several of the jurors described the man as having "a gravelly, deep voice" and that the caller addressed at least some of the jurors by name.

"He purported to be affiliated with a group of some sort," Marshall said. "I am convinced the same person made all five phone calls."

"The calls were all of a similar nature," he said, and included "pointed reference to this case." He did not elaborate.

The judge assured the jurors at the same brief session that he is also convinced "that no party to this case . . . no one connected with this case had anything to do with those phone calls."

Marshall offered no basis for that conclusion but evidently reached it after questioning each of the five jurors this morning when they reported the calls to him and again this afternoon after all of the lawyers had been notified.

Marshall also questioned the other 12 jurors to be certain they had not received similar calls without reporting them.

All of the five jurors who were called told Marshall they felt they could put the incident out of their minds and decide the case on the basis of what transpires in the courtroom.

"But all of us are human," Marshall told them, "and things can be said to a juror sitting in a case that cannot be put out of mind. I have reached the conclusion that in fairness to you, in fairness to the parties and in fairness to the lawyers that we should ask you to consent to sequestration for the balance of the trial."

One of the jurors, reportedly a woman with small children in her care, was excused, but the other 16 agreed to spend the remainder of the trial under the guard of U.S. marshals at Chicago's Bismarck Hotel.

The judge variously described the caller as "a meddler," "a crank" and "a total outsider" to the case. He told the jurors he also is convinced from what they had told him and from his experience in other cases "that this person does not really pose a threat to anyone."

But Marshall said he feared the jurors might not be able to render a fair verdict "if you were exposed to any more behavior of this kind."

The jurors were to be sequestered starting tonight even though the trial will not resume until Monday. Closing arguments are expected late next week.

Williams and his codefendants have been accused of conspiring to bribe Sen. Howard W. Cannon (D-Nev.) in 1979 in return for his help in blocking trucking deregulation legislation opposed by the Teamsters.

Marshall praised the jurors for reporting the calls to him. He described them as a startling illustration "of the fragility of the system under which we operate.

"We look at our institutions, and they appear to be robust," he said, "but actually they are kind of fragile."

The judge did not indicate what kind of investigation, if any, might be mounted to try to identify the caller. Prosecutors and defense lawyers declined to comment.