I agree. They can say they are Sallie Mae if they are a collection agency trying to pick up the debt. This is usually the case, and I personally wouldn't be giving my info out to some trashy collection agency! If you ARE having financial issues then call them using their official number. If nothing else at least you are keeping them in the loop and sometimes they can even suggest options to help you out.
Somebody might have stolen your identity and the Sallie Mae bill collectors are coming after you thinking that you're the person that they're looking for.