If you have musty, years-old debts to the Internal Revenue Service, you may start hearing from a private debt collector.

Beginning this month, the IRS has revived the use of debt-collection companies, and will start sending about 100 letters a week out to those with long-overdue federal tax accounts. If you are one of those people, your back taxes will be assigned to one of four private-sector collection agencies.

The controversial program — it's opposed by the IRS agents' union and Nina Olson, the national taxpayer advocate — will grow to 1,000 letters a week for each firm by the end of summer.

The IRS has tried private debt collectors before. Congress stopped the practice in 2009, and has now reauthorized their use. It's an experiment that will be closely watched by lawmakers.

First contact: The IRS.

Through letters from both, you'll hear from the agency and a debt collector. Only then will the dreaded phone calls begin from one of the following collection firms: CBE Group, of Cedar Falls, Iowa; Conserve, of Fairport, N.Y.; Performant, of Livermore, Calif.; and Pioneer, of Horseheads, N.Y. (Those with student loans or other debts will recognize these names).

Initially, the IRS will target taxpayers owing less than $50,000 and move up from there. Uncle Sam wants money from folks who have owed taxes for many years, and offers incentives: The debt collectors can keep up to 25 percent of what they get back for the Treasury.

Even the IRS acknowledges there's room for abuse. If you think this is not only a bad idea, but confusing – given the raft of phone scammers posing as IRS agents and preying on American taypayers – you have company.

"The IRS remains extremely concerned about the many con artists out there who masquerade as IRS employees or contractors," said Mary Beth Murphy, who heads the small-business and self-employed division at the IRS. "We urge everyone to be on the lookout for scammers who might use this program as a cover to swindle taxpayers."

Debt collectors can identify themselves as "contractors" of the IRS, but they can't bully you. They must adhere to the Fair Debt Collection Practices Act. Like IRS employees, they must be courteous and respect taxpayer rights, the agency said in a press call with reporters.

Once the IRS sends you a letter, your designated private debt collector will send its own letter and a representative confirming the account transfer. To protect the taxpayer's privacy and security, both letters are supposed to contain information on the tax amount owed.

Never send money.

The four debt collectors are authorized to discuss payment options. But any tax paid must be sent, either electronically or by check, to the IRS or U.S. Treasury.

"Here's a simple rule to keep in mind: You won't get a call from a private collection firm unless you have unpaid tax debts going back several years and you've already heard from the IRS multiple times," IRS Commissioner John Koskinen said.

"The people included in the private collection program typically already know they have a tax issue," he said. "If you get a call from someone saying they're from one of these groups and you've paid your taxes, that's a sure sign of a scam."

In addition, certain back taxes are ineligible, including accounts for minors, taxpayers in designated combat zones or with offer-in-compromise or installment agreements; deceased taxpayers; victims of tax-related identity theft; innocent-spouse cases; and people in presidentially declared disaster areas who request relief.

"Most taxpayers owing tax debt to the IRS simply do not have the ability to pay," said Michael Gillen, of the law firm Duane Morris. "Such taxpayers need to protect themselves, not from the IRS, but from scammers.

"The use of private debt collectors to collect delinquent tax debts is a potential minefield for taxpayers with valid tax debts, given the proliferation of tax scams and the overwhelming presence of fraudsters posing as IRS representatives attempting to collect debts, Gillen said. "Such fraudsters will certainly try to capitalize on the use of private debt collectors."

If you're harassed by overly friendly debt collectors, you can file a complaint online with the Consumer Financial Protection Bureau (www.cfpb.gov) and the U.S. Treasury's Inspector General for Tax Administration online (https://www.treasury.gov/tigta) or by phone at  1-800-366-4484.

And remember: Congress' mandate is not to go after those who can't pay the entire tax owed for 2016 on Tuesday, April 18, this year's filing deadline.

Fedwatcher note

Could the Federal Reserve's profitability be a factor in unwinding its balance sheet? Raymond Stone, a former New York Fed economist, is concerned that if the Fed is "too slow to unwind, they could suffer losses, which could result in congressional intrusion that could compromise Fed independence."

Last week, Stone made the fascinating case that the central bank should start to unwind all the securities on its balance sheet, and sooner rather than later, in a presentation to the Philadelphia Council for Business Economics.

Stone is cofounder and managing director of Stone & McCarthy Research Associates, headquartered in Princeton. Prior to forming SMRA in 1989, he was Merrill Lynch's chief financial economist. You can find a copy of his presentation last week online at the council's website: http://www.pcbe.org.