Statutes of limitations on debt vary a great deal from state to state, assuming you are in the US.
There are limits as to what times, what days, and how many times a day they are allowed to call you.
As ScottGem has suggested, you can write them and demand that all communications be in writing via the mail and that no phone calls are to be made to you or your employer regarding this debt.
I would write such a letter, and at the same time, request validation of the debt. Send it certified mail, return receipt requested. This way if you have to take some type of legal action against them, you will have proof of your request.
You can view the following link if you are in the US, these are the laws that govern collection agencies.
Fair Debt Collection Practices Act Index FDCPA
Good luck to you.