Dispute an $800 finance charge on my account for a phone I never agreed to finance

Dispute an $800 finance charge on my account for a phone I never agreed to finance. At no point did I sign a financial agreement authorizing this charge. Any attempt to hold me responsible for a contract I never signed is completely invalid and unacceptable. During the sale, my sales manager explicitly stated that I would receive a $600–$700 credit for trading in my old phone, and that I could call to have the new phone unlocked. This agreement has not been honored and unhelpful. It is not legally permissible to enforce a financial agreement without a signature. ALL communication WITH THEM IS A WASTE OF TIME