The number of scams like this that are out there is insane.
The only thing that I could think of to do was to not return the calls at all. I asked my mother about it, and she told me that no legit collection company would call you like that. She should know...she would as a fraud investigator for a major credit card company for 7 years. She was great at finding things no one else could.
Don't answer the calls. And, certainly don't return them. I know its annoying but its easier to hit delete than to try to explain to your phone company why you have a $1500 phone call to Guam.
You might see if your phone company has a privacy guard service. I know SBC does. I have it and had to turn it off while my husband was over seas so he could get through. I turned it back on just recently...no annoying phone calls.