Something like this?
Dear afni,
Thank you for your investigation into this matter. As for what I specifically dispute, it is the alleged debt in its entirety. My first letter was a request for debt validation, as such you are required to provide documentation, since you reporting this account to the credit reporting agencies in the first place, I can’t conceive of why you need any type of documentation from me.
I require all supporting documentation to substantiate your claim. Examples may include but is not limited to, a copy of the original agreement and/or contract signed by me, a closing statement with total monies owed, a breakdown of what monies are owed and for what, documentation that proves your right to collect upon this account (such as a transfer or a bill of sale, etc.)
I am sure if you are collecting a legitimate debt then you should have already, or have access to these important documents.
There is no need for you to have a current phone number for me as I have no wish to speak to any of your representatives. All correspondence with me is to be through the United Sates Postal Service.
Thank you for your time and consideration of this very important matter