>When I pressed about the previous visit, they said they submitted it over 7 months late.

Then they should be SOL. Call your insurance company again and reiterate that an in-network provider (if that's what they are) is trying to collect from you after they failed to file a timely claim despite having your information. That should get them to bring the hammer down and settle this on your behalf.

If not, contact your state health insurance regulator.