Demand letters with no support(They want to rip people off)

Ampath sends me demand letters for reference numbers I am not aware of. When I request statements at, Accounts@ampath email address, for all my accounts with them they only send me one(the one I am currently paying and am aware of). For the rest they send demand letters and not statements. How do I pay based on a demand letter that is not accompanied by a statement? When I call them, their lines are busy, I hold for long. I am trying on my side and they are not coming to the party. All they do is threaten. They are harrassing me and I will take this matter forward. This is emotional distress!!!!!!!!!!!!!!!!!!!!!!!