Senate hearings begin this week on several proposals designed to change the way thousands of government workers, ranging from scientists and engineers to secretaries and auditors, are paid.

Two of the bills before the Senate Civil Service Subcommittee are designed to give agencies greater salary flexibility. One, by Sen. Jeff Bingaman (D-N.M.), would unhitch thousands of government scientists and engineers from the 15-grade pay system used for 1.6 million white-collar civil servants. It would give agencies greater flexibility to pay workers higher salaries, based on performance. The Navy has been using a similar system for several years in the China Lake, Calif., research facility and at the San Diego Navy yards.

The second, proposed by Sen. Paul S. Trible Jr. (R-Va.), would give individual federal agencies greater authority to set higher pay rates for employes who are hard to recruit and retain or to match wages paid in local private industry. Currently, that so-called special rate authority rests exclusively with the Office of Personnel Management.

The third bill, by Sen. Dan Quayle (R-Ind.), would set up a 10,000-member independent Civilian Procurement Corps in the Defense Department to monitor contracts and purchases. The employes could be taken out of the Civil Service system under the proposal. Hearings on the bills, which could lead to pay changes for other civil servants, will be held April 15, 25 and 30. Expensive Phones

The Federal Times newspaper says that a check of long-distance calls by Housing and Urban Development inspectors shows that 30 percent of the agency's long-distance calls were for personal rather than government business.

The audit is part of a government-wide telephone study, initiated by the President's Council on Integrity and Efficiency, aimed at targeting and eliminating telephone abuse. Inspectors general in 16 agencies have been conducting the probe to identify the extent of the problem.

Many of the unauthorized calls were made after work hours. In some cases, the newspaper said, contract employes rather than regular HUD workers did the improper dialing. Tax Day Activities

The National Treasury Employees Union will hold a news conference Tuesday at 10 a.m. at the National Press Club. The union, which represents many IRS workers, says it will outline a plan to identify and collect from tax delinquents who owe the government billions of dollars.

Also on April 15, at a special 9 a.m. newsmakers breakfast, representatives of the National Committee on Public Employee Pension Systems will talk about the pending retirement plan for feds hired since 1984.

PEPS, chaired by former Civil Service Commission Chairman John W. Macy and former representative Hastings Keith of Massachusetts, says it will propose a pension plan that will save the taxpayers $500 billion over the next 40 years and still give workers benefits comparable to those available to private sector workers. Pension Tax Change

The Senate Finance Committee this week will take up the controversial proposal to eliminate the post-retirement, tax-free period now available to federal, state and local government workers and others who contribute to their own retirement. Under current law, pensions of those people (who have already paid taxes on their pension contributions) aren't taxed until they have recovered all the money they put into the system.