Called but left no message. Called them back. Debt collector.
They said my number wasn't listed as having an active file with them. Since they didn't leave a message, they couldn't look it up to see why they called me. Still don't know who they were looking for.
Caller from Navient, which may be monitored or recorded. I imagine if the employee wanted a call from them at work, he/she would have given them his/her direct line.