Um, it's a debt collection agency.... ie: you or someone you know owes someone money and hasn't paid them for a while(read: credit card debt, etc). this company bought the debt and now they are trying to collect the debt cause they get to keep whatever they collect minus their cost (their cost is about 10% or less of what you owe and they are looking to get 45-50% of what you owe, so they make a healthy profit...)
could be it's actually a wrong number (or it's someone you know that owes them), if that's the case just have fun with them and say whatever you want or tell em to stop calling.
if it's actually your debt, well you know the rest... ;)