It's a call center that represents multiple companies.  Ferguson Waterworks was mine and they had an automated message reminder for my appt on Thurs.  Sallie Mae may use the call center services, but the number isn't limited to Sallie Mae.

This was a legitimate call from Sallie Mae Rewards Center

Junk call from "United Health Care." Gave their number as 877-232-8621 but Caller ID showed 855-232-8621. We're on the Do Not Call list. This company is apparently a health insurance company, and of course you can't trust those slimy b*****ds. If they call you, report them to the FTC!