WASHINGTON — If you owe Uncle Sam, how can you find out if the Internal Revenue Service has hired a private collection agency to handle your case?

You will be notified by an IRS letter, according to Deborah Wolf, project director for filing and payment compliance modernization at the IRS.

"It tells the taxpayer the name of the company that they are being assigned to, along with contact information for that company as well as a special 800 number that is being used for that program only," said Wolf. "We also include a special tri-fold publication that goes into more details on the rights of the taxpayer."

Within five business days, a second letter will arrive. This one will be from the collection agency.

Taxpayers have 10 days to opt out, either by promptly paying the IRS or sending a letter asking that their case not be assigned to a collection agency.

Through the first 26,000 cases assigned to private collection agencies, only 130 taxpayers submitted opt-out letters, Wolf said.

If you don't make a prompt payment or opt out, you can expect to start receiving phone calls at home, at work or both.

The IRS does not give the private collectors your phone number, but the collection agencies have other sources of that information.

To date, the cases assigned to private collection agencies have involved back taxes of $25,000 or less owed by individuals. The delinquencies range from six months to six years.

"Payments must go directly to the IRS," said Wolf. "The collection agencies are not authorized to receive the actual payments on our behalf."

How big is the problem?

The federal government has a $120 billion shortfall of uncollected taxes, according to Sen. Charles Grassley of Iowa, ranking Republican on the Senate Finance Committee.

That doesn't include people who underreport income or fail to file a tax return.

If you think you owe money to the Internal Revenue Service, there's a toll-free number to call, (800) 829-1040.

If you have moved in the last year and you think the IRS may have not sent you your refund or has lost track of your past-due taxes, fill out IRS Form 8822 for a change of address.