A lot of collection agencies, telemarketers, and annoying people like that have those systems that Grumpy Pirate mentioned where it'll not only block their number from caller ID, but actually let them choose what number shows up. Over on creditboards, I've read quite a few times where someone's caller ID will have a friend or relative's phone number and when they answer, it's a collection agency. Sneaky. There are so many databases these people have that connect you to everyone and their brother that they choose someone who seems "close" to you and hijack their number so you won't ignore the call.
Sounds like you got someone that is just trying to mask their number.