When a company hires a 'debt collector' they CONSTITUTIONALLY forfiet their rights to that debt. You dont owe the debt collection company a red cent. You dont even have to communicate with them.
The simplest way to get rid of them is to INSIST they give you ALL of their contact information and then send them a certified letter requesting all the phone calls stop and everything be in writing from that point forward.
If they furhter insist on calling you can report them then to the Federal Trade Commision for harrassment.
This process has worked for me in the past and the unsecured debt gets written off.
(This obviously doesnt work if its a service youre curently using or its a debt with security attatched. ie collateral)