Call the collections agency. I am an attorney who works for a collection firm, when someone calls me and wants to settle, I'm happy to talk to them. It is never too late to work out a deal with them. I would say 80% of my settlements happen 5 minutes before the case is called in court. BUT, if you are confused about this debt in any way, make sure they answer those questions for you! Any attorney or collections agent should be happy to clarify anything you don't understand. If you get someone who you think is extra grouchy, ask to be transferred to someone else, or to the manager. Tell them everything you stated above and that you'd like to resolve the case. If you feel unconformable with the way they are talking to you or the way they are handling it, you may consider getting an attorney to call for you, but you certainly do not need one. I'm here if you need anything else.
Settlement offers and settlement negotiations are not admissible in court as evidence that you owe the debt. Often, the collection agency will tell you to speak with their attorney directly. You can negotiate with their attorney, too.
If you do not know what debt this is (and perhaps even if you do), you should request a FDCPA verification of the debt. Essentially, a letter that requests that they confirm the amount owed and verify that it is owed by you. Their response should also include information about the original creditor.
Since you say you need to file an answer, I am guessing that this case was filed in district court (and not small claims/justice court). In district court, once you have filed your answer you would send questions to the plaintiff (interrogatories) and requests for production of documents. You would ask for any document that evidences the assignment of the debt to the collection agency and request a description of the terms of the conveyance. If they claim a debt you owed to a rental company, they have to prove that it belongs to them before they can collect. Typically judges are a bit generous about allowing them to prove this, so don't expect much but they will have to show something.
If you do not owe this debt, you should probably meet with an attorney and fight this matter. Best of luck to you. Litigation can be frustrating and slow sometimes.
Providing this answer does not form an attorney-client relationship. Most legal questions are exceedingly fact-sensitive and therefore this answer is a best-guess based on the information you provide. You should consult an attorney licensed in your state to further discuss your matter.