What are the next steps to responding to a debt collection agency?

Backstory, I am a real estate agent located in the state of FL. Around 2 years ago in 2024 I entered a contract with Zillow’s Premier Agent program, designed to get realtors connected with clients. I entered a 6-month contract with them and the monthly cost was around $500 for me. At around the 4 month mark, I attempted to contact my sales rep and discuss ending my contract because I was unhappy with the results Zillow provided. I tried to do this 2-3 weeks before the next billing date was going to run, and he did not take any of my calls, emails, or text messages. After a week of this, I contacted the zillow support line and got through to his manager and they got things moving, he replied and said that he had been out with the flu and would get up with me very soon. Again, he stopped replying to my texts, emails, and phone calls expressing my wishes to end the contract. Finally I contacted Zillow’s customer support line again and iirc they got me on a joint call between him and his manager, well this was already after the new billing cycle had ran and I told them because of my repeated attempts to contact before the cycle ran I felt I should not have to pay that final month. They verbally agreed to this and ending my contract.

Well they ended up sending my account to collections anyways and what I thought was a number trying to reach the old owner of my number, was actually calling for me and told me of the debt I owed. I stupidly admitted the debt was mine on the phone and told them I could send proof I had this debt resolved. The email account where all of my emails with the sales rep emails ensued was deleted and message recovery is not possible at this point, so I don’t even have the original contract from Zillow to reference to. The only thing I have is my text messages to the sales rep when I was attempting to discuss what we verbally discussed over the phone, which I also have 0 record of. Wondering what to do next, do I send a debt validation letter? Or is that now negated by my admission to the debt over the phone? What may happen if I don’t pay this debt?