A jury of six men and six women began deliberations today in the trial of Teamsters union President Roy Lee Williams and four other men accused of conspiring to bribe Sen. Howard W. Cannon (D-Nev.).

Sequestered since last week because of a series of ominous phone calls to some of the jurors at their homes, the panel retired after an 85-minute lecture on the law in the case from U.S. District Court Judge Prentice H. Marshall. The jury adjourned for the day shortly before 5 p.m. here.

Marshall indicated that he expects the deliberations to last several days. In his instructions to the jurors, he told them that it was not necessary to find that Cannon, who was not indicted, accepted the bribe or even that he took any official action as a result of the alleged offer.

"It is the agreement among the defendants to offer Sen. Cannon a bribe in order to influence his official conduct which is charged, not the receipt of any bribe or the performance of any act by Sen. Cannon," the judge said.

At the same time, Marshall emphasized that "entering into a business venture with a public official in and of itself is not a criminal offense. In the absence of a conspiracy to bribe, it is not unlawful to consider investing in real estate with a United States senator, with persons associated with a senator, or with anyone else."

The key question confronting the jurors is what happened in Cannon's office in Las Vegas on Jan. 10, 1979, when Williams and a longtime Teamsters union ally, Chicago insurance executive Allen Dorfman, arrived with a Teamsters lawyer from Washington to discuss deregulation of the trucking industry, which the union opposed.

The government has charged that after the lawyer left, Dorfman and Williams "cut a deal" for Cannon's help in sidetracking deregulation legislation that was expected to be introduced by Sen. Edward M. Kennedy (D-Mass.). In return, Cannon was allegedly promised exclusive rights to buy a piece of Teamsters pension fund property in Cannon's Las Vegas neighborhood for $1.4 million.

The indictment relied on FBI tape recordings compiled as the result of a separate investigation of Dorfman that started on Jan. 30, 1979. The tapes show repeated discussions of "a firm commitment" to Cannon and of the frustrations that Dorfman and Williams encountered in making good on it.

Although owned by the Central States Pension Fund, the property was being disposed of by an independent company hired in 1977 under Labor Department pressures to take charge of the controversial pension fund's assets.

The property eventually was sold to another party for $1.6 million and Cannon, who was defeated for reelection last month, later introduced a trucking deregulation measure that Congress enacted in 1980. But the government charged that until late May, 1979, the defendants enaged in "frantic efforts" to get other bidders to withdraw and nail down the property, at first for Cannon and his neighbors and later for Cannon alone.

Williams and Dorfman contended at the two-month trial that they only promised the senator "a fair chance" to bid on the property. Cannon testified that he had not even been promised that, but, as Judge Marshall observed at one point, the senator was contradicted by every other witness who testified about what happened on Jan. 10.

The prosecution played 50 recordings at the trial. Cassettes of each conversation, along with other exhibits in the case, were furnished to the jury when it retired.

The indictment charges each defendant with conspiracy to bribe a senator and 10 related violations of federal law, including wire fraud and interstate travel in furtherance of the bribery scheme. Each count carries a five-year prison term.

In addition to Williams and Dorfman, the defendants are: Joseph (Joey the Clown) Lombardo, a reputed Chicago mobster and frequent visitor at Dorfman's insurance agency in the Teamsters pension fund building here; Thomas F. O'Malley, a trustee for the pension fund, and Amos Massa, a former trustee.