Washington CNN  — 

Thousands of Internal Revenue Service employees volunteered to go back into their offices Monday after the agency offered incentive pay to those who handle “mission-critical” work.

That includes answering calls from taxpayers and opening mail – which has been piling up in trailers since the government work-from-home orders took effect last month.

The workers are spread across 10 different locations around the country.

All workers who report in person will see at least a 10% increase in base salary for at least the next four weeks. Those who are reporting to mailrooms – which have been deemed higher risk – will get a 25% boost, said Chad Hooper, the national president of the Professional Managers Association. The group represents about 30,000 non-union IRS workers.

Most IRS employees have been working from home during the coronavirus outbreak, even as Congress has added to the agency’s workload. Not only is it the middle of tax filing season, which was extended until July 15, but the agency is also tasked with sending out economic stimulus payments that were approved by Congress in late March. The IRS has sent more than 88 million payments so far.

But the already-strapped agency has mostly stopped answering taxpayer phone calls, urging those with questions about the stimulus payments not to call. It also stopped processing paper returns, including amended returns, because no one is there to open the mail.

A majority of those employees returning will bring the call center back online. They will prioritize speaking with taxpayers who are required to call to verify their identities before their refunds can be processed, Hooper said.

An email sent to staff last week said that workers must keep their faces covered while in the buildings, but would have to bring their own masks – drawing criticism from some House Democrats, who said it was “irresponsible and unethical for the IRS to demand those workers obtain their own protective equipment.”

Hooper said on Monday that the IRS had obtained enough face masks over the weekend to issue one to each worker, but they still must have arrived at the locations with their faces covered.

“The IRS will continue to do everything possible to protect employees while also providing important services and assistance to the nation’s taxpayers,” the agency said in a statement on Saturday.

It did not immediately respond to further questions from CNN on Monday.

The National Treasury Employees Union, which represents IRS workers and federal employees in 33 other agencies, said it is communicating with the IRS to make sure there are adequate cleaning and disinfecting supplies at locations where workers have returned.

In a statement, union President Tony Reardon said there are no immediate plans to pull the thousands of employees successfully working from home.

“NTEU believes they should remain on telework until all state and local stay-at-home orders are lifted and health and safety precautions have been met,” he said.

CNN’s Kevin Bohn contributed to this report.