Got a question for the Internal Revenue Service? Keep calling, and be prepared to wait.

With the April 15 tax filing deadline around the corner, an estimated 18 million callers seeking help from the IRS won't be able to reach a real person this year. That's about 40% of all tax season callers, according to IRS data.

Those that manage to get a person, might have had to wait as long as 25 minutes.

And lines at walk-in taxpayer assistance centers are also long; some have reported 90-minute waits.

"Millions of taxpayers are finding unacceptably long wait times on the phone and at our taxpayer assistance centers to get basic questions answered," said IRS Commissioner John Koskinen at a House panel this week on the IRS budget.

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Koskinen mostly blames ongoing federal budget cuts. The IRS budget is $850 million slimmer than it was in 2010. And there are 6 million to 8 million additional taxpayers since then, he said during the hearing.

Another underlying problem is that the IRS has lost nearly 10,000 employees since 2010, much of it due to retirements.

The problems were exacerbated this year by a 10-day delay in the tax filing season due to the 16-day federal shutdown in October, said Colleen M. Kelley, president of the National Treasury Employees Union.

So far this year, the volume of phone calls is similar to 2013, Koskinen said. Yet, callers are asking more complicated and time-consuming questions about Obamacare and new rules that force foreign companies to report more offshore investments by U.S. taxpayers.

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On average, the IRS is on track to answer 60% of calls this year with wait times averaging between 15 and 25 minutes, Koskinen said. In 2011, the IRS answered 70% omf calls and the wait time was only 13 minutes, according to a report by the National Taxpayer Advocate office.

"We think it's unfair to our taxpayers not to provide them with the level of service we all think they deserve," Koskinen said at the hearing.

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Budget cuts last year forced the IRS to freeze hiring and cut 85% of training and travel, Koskinen said. The agency's 2013 budget of $11.29 billion remained the same this year, a belt-tightening period for all federal agencies, thanks to the so-called sequester.

Existing staffers, especially those manning the phones, have said in surveys that they're feeling the pressure of lack of manpower and increased workload, according to the National Treasury Employees Union, which represents employees at the federal tax collection agency.

"When taxpayers do finally get through, they're usually very angry and frustrated and they take it out on the IRS customers service representatives trying to help them out," said NTEU President Kelley. "It's really made the jobs of the front line assistants a lot more difficult."