Most anyone who has been in collections knows the tricks. First off request in writing documentation that the debt is valid. Most third-party collectors do not have this. If they cannot produce documentation that the debt is valid, they cannot collect it. Secondly, they must tell you, every time they contact you, that..."This is an attempt to collect a debt and all information you give them can be used to collect the debt". If they do not say this every time then they are in violation of the FDCPA (Fair Debt Collection Practices Act) and they can be sued by you.
There are a lot of rules and restrictions that third party collection agencies must abide by or they can be in trouble. It would be smart for you to study up on the FDCPA.