Complaint / Review
Receivables Performance Management
Callers Claiming to Be Sprint Are Not, Call Sprint Before Paying

Do not pay these people calling you saying they are from Sprint. They are not. RPM is false.
Call Sprint and ask them if you have a bill. This company is a scam. If you do owe "only" pay to Sprint, not over the phone to these people who say they are sprint.

And never give your cell phone to these people, if they were from sprint they would have your number already. Once they get your money they never send it to Sprint.
Then call a lawyer if they do not leave you alone.
Also have them send you in writing what you owe if Sprint say you do not owe.