Is it legal for debt collectors to use intimidation tactics when contacting the debtor?

NO!NO. They are now allowed to harass you. You should call back and speak with their supervisor. If they don't do anything, report them to consumer affairs. The Department of Consumer affairs works at the state level- so contact your local Consumer Affairs department to file a complaint. In Addition No.. and if they do, tell them your contacting the FTC- Federal Trade Commission- to report them.

Debt collectors, in the US, who cross the line are actually breaking the law (5 laws in particular):

1. Fair Debt Collection Practices Act
2. the Fair Credit Reporting Act
3. Truth in Lending Act
4. Fair Credit Billing Act
5. and the Gramm-Leach-Bliley Act