Question regarding vaildation

Long story short: Somehow T-mobile got it in their head I have had an account with them. The claim I owe them $165. I have never had a T-mobile account in my life. I was clueless about this until a creditor contacted me. I asked for validation within the proper time frame and they sent me single statement that said $132. I sent them a letter back stating that they had failed to validate the debt, don't report it, etc. All of this is documented, return receipts, etc. I checked my credit report and they have reported it.

My questions: What constitutes proper validation? In a perfect world, a signature on a contract would be required and I know I would be golden there, because I have never and would never sign a contract for a cell phone with anyone. (I don't use it enough to pay for an expensive plan.)
Also, shouldn't any sort of validation reflect the actual amount they are claiming you owe? How can a debt be valid without some sort of accounting that shows the amount they are claiming you owe?
Thirdly, the debt is not listed as "in dispute" on my report.Should it be or did their piss poor showing count as validation?
Finally, would I have a reason to sue for this?