I think people need to be thinking more along the lines of phishing rather than debt collection. Following the mortgage crisis recently there are a lot of people in bad debt situations, so people are thinking this to be legitimate but...
I wrote on another forum:
I don't think this is connected to a collection agency. I have had my cell number for many years,I also happen to have rather good credit (not gloating)and I have been receiving calls from this number for about two weeks now. The first time, I answered, it was sombody speaking broken spanish I think, I could understand part of what they said. The person was asking for someone by name, I could not quite get it, after about ten seconds I simply told them that they must have the wrong number and then they hung up. I have been receiving a call almost daily since... and I have not answered it since. Nor do I plan to, because like most people I have a message on my phone that should make it fairly clear that they are calling the wrong number without the fact that I already told them so once