It is illegal and they know it. The next time they call tell them you are recording the calls from now on. I would definitely start recording them because if they don't take you seriously you can sue them and win with the recordings. You could be swimming in some serious money if they continue their stupid ways. I know this because I used to collect accounts for a living and I know what you can and can't do.
My number was owned before from this lady who makes it a profession to owe money and then have my number called from collection agencies. I am so upset!! How can I get this lady to stop using my number and those flipping agencies stopping to harass me?