The Internal Revenue Service is about to start using four private debt-collection companies to chase down overdue payments from hundreds of thousands of people who owe money to the federal government, a job it has handled in house for years.

Unlike I.R.S. agents, who are not usually allowed to call delinquent taxpayers by telephone, the outside debt-collection agencies will have free rein to do so. Consumer watchdogs are fearful that some of the nation’s most vulnerable taxpayers will be harassed and that criminals will take advantage of the system by phoning people and impersonating I.R.S. collectors.

Additionally, one of the four companies that the I.R.S. has hired, Pioneer Credit Recovery, a subsidiary of Navient, was effectively fired two years ago by the Education Department from its contract to collect delinquent debt for misleading borrowers about their loans at what the department called “unacceptably high rates.”

Proponents of the plan, who include Democrats and Republicans, point out that the debtors are shirking their tax obligations and that collecting from them will add to the Treasury’s coffers. The congressional Joint Committee on Taxation estimated that the new debt-collection program had the potential to gain a net $2.4 billion over the next 10 years.