There were similar frustrations for those who called the agency’s so-called balance-due line in hopes of making payment arrangements for taxes they owed. Fewer than 7 percent of such calls were answered, and the typical wait for those that were stretched to more than 80 minutes.

I.R.S. officials said they were reviewing the taxpayer advocate’s findings.

“We are continuing to assess the impact of the shutdown on our various operations across the agency and remain proud of the many I.R.S. employees who have risen to the resulting challenges,” the agency said in a statement. “The I.R.S. is committed to continue making improvements across our information technology, tax enforcement and taxpayer service operations.”

Data released by the agency last week showed that Americans had filed 12 percent fewer returns through Feb. 2, compared with the same point in 2018, and that the I.R.S. had processed 26 percent fewer returns. The agency has given no indication — in Tuesday’s statement or otherwise — that it might consider pushing back April’s tax filing deadline in order to account for shutdown-related snags.

The taxpayer advocate’s audit did not focus solely on the shutdown’s impact and reflected an effort by the office, which is led by Nina E. Olson, to capture the typical taxpayer experience when dealing with I.R.S. The audit said many Americans’ feelings about the process could be boiled down to two words: “extreme frustration.”

The audit found that a policy change made by the Trump administration intended to simplify the tax-filing process — the creation of a “postcard size” 1040 form — has made filing more difficult because the new form does not include much of the information that many taxpayers need to complete their returns.