I keep getting calls but there is no phone number just '513' comes up on the phone.

I kept getting these calls as well. I never answered until today. They never left a message, which should have been my first clue. Turns out, at least in my case, that it is yet ANOTHER phone number "disguise" for Sallie Mae (SLM Financial).

yeah, Collections of some sort I would presume.

I've been getting calls from this number. I have no idea why Sallie Mae would be trying to call me. I do know however, that such a number is absolutely impossible to actually occur. This means that 513-000-0000 is a Caller ID spoof. This is actually against the federal law. I told them last time that the next time they call and it comes from a spoofed caller ID number, that I'm turning them into the FCC.

This could be anything.  I live in this area code and this is the number that comes up when my doctor's office calls me.  I think it's just because there are so many extensions that the system doesn't pick it up.  When my husband calls from his office it only shows the extension number of his office.  Ie. comes across as Unknown 6789