I believe it is Sallie Mae. They call from "cell" numbers frequently. I try and call back and it says the line is not a working line and on this number imparticular they said the voicemail box had not been set up.

I receive calls from this number on my cell phone and when I call back, they deny that anyone from their "house" ever called my number.  It is a Verizon land line from New York, NY.  If they continue to call, I intend to contact the police to file a harassment complaint.  Once the police contact Verizon, their phone should get disconnected, at least temporarily.  Problem solved - I hope!