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...