866-539-2764

Receive plain letters in the mail saying to call this number to reduce debt. doesn't say anything about who they are or what kind of business they conduct.

Any communications received by US mail which appears fraudulent should be taken down to the local post office so that the postal inspectors can take a look at it.  THis could be mail fraud of some sort

Well, that is pretty clear.  They want you to pay them money up front (illegal) to do what you can accomplish for free