The advice above is somewhat solid. However, verifying that the initial correspondence was sent is not something that occurs in a CRA dispute.
Disputes with the CRAs are solely for the validity of the debt. You lodge the dispute, the CRA asks the creditor for validation of the debt. They are not going to to ask for validation that you received the initial communication.
So the dispute process kind of serves as your second chance to request verification of the debt, but it will not involve an investigation into the actions taken by the collection agency.
If it comes out that the debt is verifiable and valid, you're pretty much stuck. A failure to send the initial communication is kind of a "harmless error" at this point, because it wouldn't have turned out in your favor.
However, it is still and FDCPA violation either way. But the only real way you're going to get anywhere with this is to take the collector to court, where the burden would be on you to prove that you never received the initial correspondence. You would be attempting to prove a negative, which is all but impossible, for a likeley award of about a hundred bucks.
I would certainly reccommend filing the dispute. It's free and at least the debt will show up as "disputed" on your credit report.
Best of luck!