As usual, I just pick up the phone and don't make a sound - I let them make the first move. They usually say "hello," and then hang up. Otherwise, I'll answer and say, "Bueno," and they usually ask me something in English, to which I reply, "No entiendo." or "no vive aqui."
Since I NEVER give my cell number out - even to most of my girlfriends - I don't usually have to worry about this. I can usually isolate what debt or obligation this pertains to if I know the few businesses that have my cellphone number.