IRS headquarters in Washington. Andrew Harrer | Bloomberg | Getty Images

As the partial federal shutdown enters its 24th day, opening day for tax season is rapidly approaching. Accountants warn it won't be fun for filers. The IRS, the nation's tax collector, said last week it would open the filing season on Jan. 28 and will send refunds to filers, shutdown or not. The tax agency also said it would recall a "significant portion" of its workforce, ordering them back to work without pay.

Accountants say even if the taxman is able to crank out taxpayer refunds, the shutdown could make the filing process more painful than usual. "It looks like getting the refund will no longer be the issue, now it's the lack of support you will get if you try to call — and it will be significant this year," said Tim Steffen, CPA and director of advanced planning at Robert W. Baird & Co. Here's what may be standing between you and your refund if the shutdown continues.

Rule uncertainty

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Accountants have been waiting for the last word on regulations that would affect clients' 2018 returns. For instance, small businesses are holding out for the 20 percent qualified business income deduction. It's a break for so-called pass-through entities, including sole proprietorships and S-corporations. Accountants are awaiting final regulations from the Treasury on this break to help clients with their 2018 returns. The shutdown has slowed that process. "I think that deduction applies to more people than we realize," said Barry Picker, CPA and co-founder of Picker & Auerbach in Brooklyn, New York. "It would be nice to have those final regulations. "You don't want to be filing amended returns," he added.

Less help

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While the shutdown has been ongoing for more than three weeks, the IRS still hasn't rolled out its contingency plan for tax-filing season. Positions that have been furloughed include personnel who will respond to taxpayer queries, according to an earlier contingency plan from the agency. The IRS gets more than 95 million calls on its toll-free lines each year, according to a 2017 report from the agency's Taxpayer Advocate Service.

Close to 40 percent of taxpayers polled by the Taxpayer Advocate Service said that calling the IRS didn't fully resolve their problem. The advocate administered a survey to 3,735 filers in February 2017. Areas with the least customer satisfaction include return preparation assistance, information on a notice and information on a refund, the Taxpayer Advocate Service found.

It would be nice to have those final regulations. You don't want to be filing amended returns. Barry Picker CPA and co-founder of Picker & Auerbach in Brooklyn, New York

This dissatisfaction will be further compounded by the fact that this is the first time filers will submit returns under the Tax Cuts and Jobs Act — a sweeping overhaul of the tax code that resulted in higher standard deductions and new limitations on itemized deductions. Even the Form 1040 has been overhauled, trimmed to the size of a postcard and accompanied by six schedules. "I'm worried that the service won't be able to respond to questions," said Troy Lewis, CPA and chair of the American Institute of CPA's task force on qualified business income. "I can't think of a time when there's been more change for individuals in the last two decades where people will be asking for answers," he said.

Amend old returns