Why do random prisoners collect call me from jail? UPDATED w/ follow-up

It seems that once a month I get a collect call on my office line from some stranger in jail. Obviously, I decline every time as I never recognize the name provided or the voice saying the name. I would write it off as a wrong number, but it happens too often for that unless my number is one digit off from a lawyer or something. Anyone have any idea why this keeps happening?

I was out of town last Thursday and Friday and had two messages waiting for me from people trying to collect call me from jail. It is almost as annoying as a telemarketer.

UPDATE: After reading the link someone posted in this thread about prison collect call scams, I was convinced someone was trying to scam me as the chances I know someone in prison are roughly 0%. After this original post, I received two more collect calls from prison and both times, the "name" provided was a mumbled "it's me." Clearly this prisoner did not actually want to be identified and just wanted me to accept the Collect Call, which I never did.

Today I called AT&T and a) verified I had not been charged for any collect calls and b) activated the "Collect Calls Block" feature they offer whereby all collect calls are automatically blocked going forward, as the name implies. Hopefully this is the end of it.