It's probably a collection agency trying to reach the person who used to have the phone number. Likely they have their phone number blocked or wrong on the caller ID, but if you talk to them you might be able to get rid of them. Act nice and find out what company it is, then if they don't stop calling at your request, they are breaking The Fair Debt Collection Act and you can file a complaint with either the Federal Trade Commission
http://www.ftc.gov or with your state's Attorney General's Office (or both) which will very likely get them to stop.
Wait, I did come home from work, didn't I...?