Verizon sent my "debt" to collections, I don't believe I actually owe them the money that they are asking for and they haven't told me why I owe them. [AZ]

As the title states, Verizon claims that I owe them $87 as of February 2nd 2016. I paid them in full January 28th to which they confirmed over the phone. Since February they have sent it to the collection agency who has sent me mail and called me once about the debt. They also bumped it from $87 to $91 and some change.

I'm concerned it's affecting my credit score and I would like to file a small claims court case against them and I'm not sure how to proceed.

TL;dr don't want to pay a debt Verizon can't prove. How do I submit for a small claims case?

Edit: a word