Sent to collections for a balance I do not owe that should have been handled years ago

I have been sent to collections for a balance I do not owe. 

I addressed this immediately after the phony charge first came up. I communicated with online billing agents several times, each of who told me I had nothing to worry about and that they would credit back my account. I ended my service with Xfinity in January 2024 and have paid off all my legitimate bills. Clearly their system ran into a glitch over a transfer of service I had scheduled (and paid for) but ended before it had started since I never ended up moving into the new address. Inexcusably no one ever fixed the initial error despite repeated promises.

Conveniently, now, more than two years later, I can't even access my billing information and your customer service rep (that I reached only after being bounced back and forth between five other agents in different departments) tells me they are "missing information" and still cant verify what the original charge was for. I then love how the rep wanted to turn the responsibility back on me, accusing me of not calling when the issue first surfaced, which the lady couldn't seem to hear that I HAD. Never mind that Xfinity makes you go through the whole runaround to reach someone that can actually help. 

This is a failure on Xfinity's billing and customer service team. They like to avoid resolving issues they create and drag out matters so proper document verification isn't available anymore. Please fix this or I will be looking into legal advocacy.