Consumers energy Signed up for appliance service plan without my authorization

I was informed recently that I have been charged for two years on my gas bill for an appliance service plan I had no knowledge of. I was advised by an associate that she reviewed the call and that I authorized them to start a free month trial and after that I would have to call to cancel. I never had that conversation, agreeing to a service plan even for a free month, so I politely asked for a copy of the recording. She told me I could have it if I got a subpoena. I also asked for the text of the conversation, which she also refused to give me. So all I have is her word that I agreed to an appliance service plan and no details about how I did other than via a phone call. These monthly charges over the past two years come to more than $600! I had confidence in this company; that's why I didn't check my bill more carefully, but that was clearly a mistake. If they had proof that I agreed to this plan, why don't they provide the audio of the recording without requiring a subpoena?