With a deadline looming in just three weeks, Medicare drug plans are expecting a surge of new enrollment that threatens to overwhelm already busy phone lines and leave beneficiaries struggling to figure out how to sign up for the new plan.

The telephone complications are among the latest round of problems that have left thousands of beneficiaries with large belated premium payments and raised questions about whether insurers are complying with federal standards. 

Callers to Humana, one of the largest Medicare carriers, often had to wait 30 minutes to reach a customer service representative last week. Federal standards say that 80 percent of calls must be answered in 30 seconds. Test calls to several large insurers suggest that they frequently miss that goal, though the waiting times are much longer for Humana. 

Premiums, averaging $25 a month, are lower than expected. But many beneficiaries are perplexed by the government's failure to withhold premiums from their Social Security checks, an option for beneficiaries. Some insurers are demanding immediate payment of four months' premiums at once, causing hardship for some beneficiaries.

Federal officials have warned insurers about "bait and switch" tactics after finding that some Medicare drug plans did not provide the public with complete, accurate information about the drugs they covered. The inaccuracies, discovered through federal monitoring and supervision of drug plans, could discourage enrollment of sicker beneficiaries with higher drug costs. Conversely, some beneficiaries might sign up without realizing how difficult it would be to get the drugs they need.

The program got off to a rocky start in January, as tens of thousands of people were overcharged or unable to get essential medicines. The initial chaos faded in February and March. Millions of people have enrolled. Many say they have begun to see tangible savings.

President Bush often cites those testimonials as he stumps the country, urging people to sign up. Administration officials and community groups plan an all-out push to encourage enrollment before the May 15 deadline.

Michael O. Leavitt, the secretary of health and human services, said people should "act soon to avoid any last-minute rush." But at some companies, it appears the rush has already begun.

In a tape-recorded message on its toll-free telephone line, Humana says: "We are getting more calls than usual at the moment. We apologize for your wait."

Thomas T. Noland Jr., a spokesman for Humana, said, "The large amount of interest in our prescription drug plans, as the May 15 deadline approaches, means that the wait times are longer than we would like." 

Humana has been flooded with calls from people seeking low-cost coverage. In its phone message, the company says, "Humana has the lowest prescription drug plan prices in most areas." 

A federal contractor is making thousands of calls to insurers to measure the performance of their call centers. Each insurer has received data on its own performance. In many cases, the reports say, federal standards were "not met."

Medicare officials had said the data would be publicly available before May 15. Insurers are lobbying against disclosure, saying the federal standards are too stringent.

Dr. Mark B. McClellan, administrator of the Centers for Medicare and Medicaid Services, said federal officials would consider the performance of insurers in deciding whether to renew their contracts with Medicare next year. In addition, he said his agency could impose civil fines or take other enforcement action against insurers that were "substantially out of compliance" with federal standards.

Of the 42 million Medicare beneficiaries, 30 million have some type of prescription coverage. That includes 8.1 million who have voluntarily enrolled in free-standing drug plans, 5.8 million low-income people automatically enrolled by the government, 5.8 million in health maintenance organizations and 6.8 million retirees with coverage from former employers. The remaining 3.5 million have drug coverage from the military health plan or the Federal Employees Health Benefits Program.

Many Democratic members of Congress are pushing legislation to extend the deadline for enrollment beyond May 15. But the Bush administration and insurers adamantly oppose an extension.

"If we don't have a deadline, fewer people will sign up," Mr. Leavitt said. "People need a deadline, and it's working." Current beneficiaries who miss the May 15 deadline and want to sign up later will generally have to pay higher premiums as a penalty for late enrollment.

When people sign up, they can ask to have premiums withheld from their monthly Social Security checks. But the government says some requests have been hung up in federal computer systems.

Some beneficiaries said they were being billed for several months' worth of premiums that were supposed to have been withheld from their Social Security checks. For example, AARP's drug plan, insured by UnitedHealth Group, is billing tens of thousands of beneficiaries.

In a typical letter, the insurer says, "We have not received payment for your AARP MedicareRx Plan premium." Then it warns, "If we do not receive payment by 5-31-2006, we will have to disenroll you."

Jerry D. Slaughter, 74, of Castle Rock, Colo., received such a letter demanding payment of $109.04, equal to four times the monthly premium of $27.26. His wife, Beverly M. Slaughter, said the letter was "surprising and disconcerting" because she and her husband assumed that the premiums had been automatically withheld from his Social Security checks, as he had requested.

Steven E. Hahn, a spokesman for AARP, said UnitedHealth was handling the problem. UnitedHealth said federal officials were responsible. Medicare officials said Social Security was responsible.

Mark Hinkle, a spokesman for Social Security, said, "We have processed every request for withholding that we received from Medicare."

But Medicare officials said that some requests had been repeatedly rejected by Social Security computers. In a recent memorandum to insurers, Thomas Hutchinson, a senior Medicare official, reported "higher-than-expected rejections of withhold transactions by Social Security."

In other cases, the government has withheld three or four months of Medicare premiums from one month's Social Security check. 

Margaret Jarvis, a spokeswoman for Blue Cross and Blue Shield of Texas, said her company was receiving calls from customers irritated to see their Social Security checks sharply reduced. 

Jacqueline B. Kosecoff, chief executive of Ovations Pharmacy Solutions, a unit of UnitedHealth, said her company had sent letters to about 225,000 people whose premiums were not properly paid or withheld. "We will try to resolve this with our members one by one," Ms. Kosecoff said.

The government encourages beneficiaries to compare drug plans by using a Web site, medicare.gov, which displays information supplied by each insurer. 

To win federal approval last year, insurers promised to cover "all or substantially all" drugs in six categories, including cancer medications, H.I.V. drugs, antidepressants and antipsychotic drugs, for schizophrenia and severe bipolar disorder.

But federal officials found some of these drugs missing from the lists submitted by insurers for posting on the Medicare Web site.

In a recent memorandum, Medicare officials told insurers to add the "missing drugs or dosage forms" to their lists of covered drugs.

Babette S. Edgar, a senior pharmacist at the Medicare agency from 2004 to January of this year, said the government had been asking insurers to correct their formulary files since October. "They have had more than six months to do this," she said.

Ms. Edgar said beneficiaries could be adversely affected if they relied on the incorrect information to select a drug plan, not realizing that all plans had to cover virtually all drugs in the six categories. 

"Beneficiaries may pass up the plan that would be best for them because they mistakenly believe their drugs are not covered," Ms. Edgar said. "They may choose a more expensive plan."

Federal officials also found that insurers did not always disclose the techniques they used to limit access to drugs — for example, by limiting the quantity of pills or by requiring doctors to get advance approval.

Medicare officials told insurers they must fully disclose such restrictions. "This protects the beneficiaries from any activity that could be perceived as bait and switch," their memorandum said.

Irvin L. Muszynski, a lawyer at the American Psychiatric Association, said many doctors and patients had had difficulty getting antipsychotic and antidepressant medications. 

Dr. Jeffrey Geller, a professor of psychiatry at the University of Massachusetts Medical School, said, "The combination of quantity limits and dosage limits makes it impossible for some patients to get the full amount of the medications their doctors prescribe."