When you received the summons, you should have also received a complaint. The complaint will tell you what they are suing you for. In your response, you can merely deny that you owe the debt and force them to prove the debt or you can admit that you owe something but that you dispute the amount. In most cases, I would advise a general denial and have them prove the amount of the debt and that you are the correct people to sue.
The usual response to a Complaint is to file an Answer. Unless you have experience representing yourself, you should have an attorney prepare it for you, as you could lose some of your defenses if you do not include them in your Answer. You can also file a Motion to Dismiss, see Arizona Rule of Civil Procedure 12(b).
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