They could be calling about a relatives' or friends' account. In my case they were calling about my brother and left a message saying he had used me for a reference.
They left an 877 phone number and a "reference" number. The reference number is something creditors use, so I asked my brother. He said NOPE, he'd never use me as a reference unless he asked me FIRST.
This is a collection company for Direct loans .. Department of education. I just canceled my payment plan because I got laid off work and now they will not leave me alone