I did this a few years ago when I had a similar problem with a collection agency that was looking for a former renter. I changed my answering machine message to the intercept tone sequence, followed by a scratchy message that sounded similar to the telco's automated message:
"We're sorry, your call cannot be answered at this time. Please check the number or contact the operator... (3 second delay) Or if you're a friend just leave a message at the beep." *BEEP*
My friends thought it was hilarious and the collectors usually hung up long before the part about leaving a message kicked in. It took a couple of months, but the calls finally stopped.
Now a friend of mine who was being harassed took a different track. He would answer the phone and listen politely to find out who they were looking for. Then he would start yelling at the collector, claiming he was owed a large sum of money from the same person. He would accuse them of being a friend of the debtor, demand they reveal where he was and threaten to sue them to get his money back. The calls stopped soon afterwards.
Your mileage may vary, etc. etc.