Per the FDCPA:
§ 804. Acquisition of location information [15 USC 1692b]
Any debt collector communicating with any person other than the consumer for the purpose of acquiring location information about the consumer shall --
(1) identify himself, state that he is confirming or correcting location information concerning the consumer, and, only if expressly requested, identify his employer;
(2) not state that such consumer owes any debt;
(3) not communicate with any such person more than once unless requested to do so by such person or unless the debt collector reasonably believes that the earlier response of such person is erroneous or incomplete and that such person now has correct or complete location information;
§ 805. Communication in connection with debt collection [15 USC 1692c]
(b) COMMUNICATION WITH THIRD PARTIES. Except as provided in section 804, without the prior consent of the consumer given directly to the debt collector, or the express permission of a court of competent jurisdiction, or as reasonably necessary to effectuate a postjudgment judicial remedy, a debt collector may not communicate, in connection with the collection of any debt, with any person other than a consumer, his attorney, a consumer reporting agency if otherwise permitted by law, the creditor, the attorney of the creditor, or the attorney of the debt collector.
§ 806. Harassment or abuse [15 USC 1692d]
(5) Causing a telephone to ring or engaging any person in telephone conversation repeatedly or continuously with intent to annoy, abuse, or harass any person at the called number.
Here's a link to a sample C&D letter. You'll need to modify it a little to fit your situation:
http://www.creditinfocenter.com/forms/sampleletter7.shtml