Urgent care billed me after I paid in full. Now they’ve sent the bill to collections. What do I do?

So I went to urgent care in 2023 after my knee went out of joint suddenly. I asked about payment options and chose to pay the full amount for an exam. The doctor just advised the RICE method, I paid, was given a receipt, and I was on my way. Maybe 7 or 8 months after, they started sending me bills for $25 in the mail. I tried calling but I was placed on hold and nobody picked up. I forgot to keep calling because I had to work. Now over a year later and I get a letter from a collections agency they sent it to. What can I do? I'm so upset and irritated at that entire company! What's the point of paying in full if you're going to charge me again after??? So what can I do? I really don't want to pay and frankly, I don't make enough to pay them for charging interest on a zero amount.