calling house we used to own

Basically all they can do is identify themselves and their employer if the person that answer asks who the employer is. They can call but they can not discuss details about your account. They can ask for you and your current contact info but no one is under any obligation to provide that info to the collector. The collector can not identify that he is a collector or state that the consumer owes a debt. In general they can not contact that number again once they know it is not the correct number. The following is the excerpt from the fdcpa that covers what you have asked:

804. Acquisition of location information
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;
(4) not communicate by post card;
(5) not use any language or symbol on any envelope or
in the contents of any communication effected by the
mails or telegram that indicates that the debt collector
is in the debt collection business or that the communication
relates to the collection of a debt; and
(6) after the debt collector knows the consumer is represented
by an attorney with regard to the subject debt
and has knowledge of, or can readily ascertain, such
attorney???s name and address, not communicate with
any person other than that attorney, unless the attorney
fails to respond within a reasonable period of time to
the communication from the debt collector.