WASHINGTON — Insurers offering prescription drug coverage through Medicare routinely failed to provide accurate and complete responses to questions posed over the telephone by federal investigators.

Many of the 43 million people eligible for Medicare drug coverage rely on insurers’ call centers for information about what plan would best meet their needs. Investigators made 900 calls to 10 of the largest drug plan sponsors. They got a complete and accurate answer only a third of the time.

The overall accuracy and completeness rate for the centers ranged from 20% to 60%. Only one insurer gave the proper, full answer more than half the time.

The investigators said that insurers answered the large majority of calls with minimal delay, were courteous, and many offered helpful suggestions. Yet, they were unable to answer 15% of the questions posed. Further, operators within the same call center sometimes provided inconsistent answers.

“Our calls to 10 of the largest (drug plan) sponsors’ call centers show that Medicare beneficiaries face challenges in obtaining the information needed to make informed choices,” concluded the Government Accountability Office in a report released yesterday.

Officials with the agency that oversees the drug program disputed the analysis. They said the report was based on “inaccurate, incomplete and subjective methods” that limit the report’s relevance and validity. But the GAO maintained that its methods are sound.

“The lack of accurate and understandable information for our seniors has been a chronic problem since the beginning of Medicare Part D,” said Rep. John Dingell, a Democrat of Michigan, and ranking member of the House Committee on Energy and Commerce. “Senior citizens are being hurt by the indifference and incompetence of the Bush administration and its friends in the insurance industry.”