As an ISP, are we responsible for answering to copyright cases that are not issued through a subpoena?

Generally speaking, you do not have to provide information to a DMCA complainant except pursuant to a lawful court order. However, even without a court order, you must respond appropriately to a DMCA takedown notice (i.e., by taking down the allegedly infringing material) in order to continue to receive the benefit of the safe harbor.