It’s nice to be called essential, said Internal Revenue Service employees recalled to work in Oakland on Thursday, but it would be even nicer to get paid.

Dozens of IRS workers arrived at the agency’s headquarters on Clay Street, none too happy about being told they’d be paid with an IOU — a form of payment that the IRS does not accept from taxpayers.

“It’s not fair,” said IRS customer service representative Susan Adelantar. “I wish I knew when I was getting paid. I’ve got some savings, so I’m OK. For now.”

The IRS workers, largely tax collection and enforcement agents and taxpayer information providers, have been furloughed since the partial federal shutdown began Dec. 22 over the impasse between President Trump and congressional Democrats on whether to fund a border wall. This week, 46,000 of them nationwide were declared “essential workers” and ordered to return to their jobs without a paycheck, so they can deal with the crush of income tax returns that will continue through April 15.

The callbacks are part of a Trump administration pattern of bringing in employees without pay in areas where their absence would be especially obvious to the public.

The IRS workers will be processing people’s tax refunds. National Park Service employees were recalled to keep Joshua Tree National Park open. The Agriculture Department brought back employees to process loans for farmers, and the Federal Aviation Administration recalled engineers and inspectors to help keep planes flying.

When the shutdown began, 420,000 federal employees were deemed “essential” and ordered to work without pay, and another 380,000 were sent home. Congress has passed legislation to give all of them their back pay when the shutdown ends. An unknown number of federal contractors also are furloughed, however, and they have received no such guarantee.

Many IRS workers are members of the National Treasury Employees Union, whose leaders said in a statement that their members were “upset about reporting to work without pay.”

“Many (at the Oakland office) are traveling from afar (Sacramento, Manteca, Vallejo, South San Francisco) and are expected to be at work although they have little to no funds for gas and bridge toll,” the union statement said.

The union has sued the federal government, claiming the administration is violating the law by ordering tax refunds to be processed during a shutdown. A federal judge rejected its request for a temporary injunction Tuesday.

Nancy Hernandez, who answers taxpayer phone calls in Oakland, said she was “happy to be here and will be happier when I get paid for being here.”

“Who knows what kind of work has been piling up while we’ve been gone?” she said.

She also said she’s trying to find a part-time job so she can pay the bills until the shutdown ends.

Whether an IRS worker can seek outside funds to get by is not clear. Conflict-of-interest rules say workers cannot solicit funds, according to a federal union vice president who asked not to be identified.

He said he had told five union members to drop personal GoFundMe campaigns that they had established to raise money to see them through the shutdown.

Customer service representative Tamara Williams said it was “exciting to come back to work” after being furloughed.

“I love my job and I’m confident I’ll get paid for it, eventually,” she said. “My mom is helping me out right now. I’m worried about the car payments. If I miss one payment, they’re not going to take my car. Three payments, I don’t know, they might.”

Customer service agent Patrick Donovan said he was financially “OK, for a bit.”

“What we do needs to be done,” he said. “I know we’ll get paid, eventually.”

But, Donovan said, he “cannot spend an IOU at the store.”

Other IRS employees, who said they preferred not to be identified, were less diplomatic.

“This sucks,” said one arriving IRS employee, as she prepared to pass through a metal detector at the employee entrance. “Write that down. It sucks.”

Steve Rubenstein is a San Francisco Chronicle staff writer. Email: srubenstein@sfchronicle.com. Twitter: @SteveRubeSF