I often tell people that they might simply deny every allegation of the petition and put the plaintiff to the burden of proving the case. In Pennsylvania, however, there is a much more powerful method for most debt cases: “Preliminary Objections.”
Preliminary Objections are a form of motion to dismiss based on the inadequacy of the pleadings brought against you. PA Rule of Civil Procedure 1019 requires that a plaintiff bringing a lawsuit based on a writing (which every credit card is) must have the contract attached to it or else include allegations in the petition from which the whole sum of money claimed could be derived.
The only catch to Preliminary Objections is that they must be filed before answering the petition or the objections are considered “waived” (let go). If you are in Pennsylvania, then, you should strongly consider getting my Silver Bullets package (for Pennsylvania, obviously). The package also contains an extremely powerful motion to dismiss any claim for “account stated,” which is the way the debt collectors have used to try to avoid Rule 1019.