This number called me at around 8:45 pm, which is odd, if it's a bill collector because they usually don't (I thought they weren't even supposed to) call after 8 pm. It probably is a bill collector, but that's still kind of weird.
Probably Sallie Mae, though they usually call from 866-517-3083.
If it is Sallie Mae, tell them you won't be threatened by anything. The real businesses that you owe money to will take things away if you don't pay, what are they going to take back? Knowledge?