Paid off debt in 2011, all of a sudden they've decided I owe them money

Back in 2011, I had around a $2500 loan, which I paid in full (Called them, got a payoff amount, paid it). About 18 months ago I was checking my credit and noticed a negative mark from that loan company, delinquent balance or something. Called the company and asked them to remove the mark because it was paid off years prior. The lady on the phone checks the system, verified the payoff "I'll note it on the account to have it removed from your credit report".

It was never removed.

About 6 months ago, I started receiving emails and phone calls from them again, stating I owed them money again. Over the phone, they stated I hadn't paid it off in full, and it was like ~$300 delinquent, and since then it's ~$600 now. I can definitely pay the amount, but I'm 100% sure they are lying to get more money out of me.

I asked them straight up, why do I all of a sudden owe you money again after I've called to have something removed from my credit report. They stated that they have been emailing and leaving voice mails since 2011 about this debt (Both of which are a lie, I've used a gmail account and looked up the emails, only started receiving the emails around the time they started calling again, and they couldn't have been leaving voicemails because I've purposely not set up a voicemail because I hate voicemails). Told her both of those reasons why they were lying, all she said was "Well we are going to keep calling about this debt" and hung up.

Literally hung up on me.

I probably would've just cut my losses and paid it, but I'm pretty furious about it now. How should I go about taking care of this? I can not find any record of my payment to them, or that it was paid in full.