Question Regarding Retroactive Charge

About 2 years ago i changed employers and their dental benefit providers were different. I had left my original employer as part of a layoff and got an exit package. When i started my new job i prompted the dentist to bill the new provider. They attempted to do so but we were told that the previous provider coverage was still active and as primary. I called the original provider and confirmed that the benefit were still active. They also told me they had no current expiration date and that i should continue to use then as primary. At the time i assumed it was due the severance package and that coverage would terminate at the end of the year. Sure enough the coverage stopped at the end the year and i swiftly moved to my new provider as main.

Two years later i get a call from the dentist telling them they got a notice from a collector asking to pay back payment the approved and confirmed two years ago. Saying that they basically “fucked up” and wanted their money back. They dentist told me they had no choice but to bill me if they were forced to pay back. I called the insurance and the collector and explained the situation. They said they would call me back but i havent heard from them since. I have all the paperwork on how the claims were approved confirmed and paid at the time. I find it asinine they would try to collect anything at this point on something they clearly messed up on their own. There has also gotta be records of all my calls to the insurance company when i was trying to confirm what was going on back then.

Any advice on wtf should i do would be appreciated it. I ‘m not above getting a lawyer to fight them off if it comes to that because this is clearly their fuck up not mine.