Oye. Dental/medical bills. Nightmare city.
Other PPs were right. First, verify the insurance information (again) that the office is billing - don't let them brush you off, verify each and every piece of information they use to bill. Then contact your HR and verify that is the latest information. Then contact your insurance company to see if any claims have been filed. Once the insurance pays out you should get an Explanation of Benefits (EOB) from the insurance company stating what they paid and what you owe - if you don't get one, something has probably broken down somewhere.
My most recent horror story: I had a dentist bill recently that I paid as soon as they sent me the bill. 2 months later I get a "final notice" on a slightly different amount, a few dollars less, with a threatening note that they were about to send me to collections. I called the dentist's office to ask what it was for (there was no indication on the bill they sent - don't get me started on that one!). They stated it was for the work I had already paid for - but why the amount had now changed, no one could explain (seeing a pattern here yet?). Then I told them "I already paid the bill for that procedure a few days after you sent me the bill in July - I'm staring at an image of the cancelled check right now". Their answer? "NO YOU DIDN'T." I swear, that is exactly what the billing person said. Not, "oh let's see what's going on". Not, "I don't have a record of that". Just "No you didn't". After faxing the cancelled check and pushing them to research it, they finally discovered they had put the payment on my daughter's account instead of mine. At that point, they didn't apologize for their mistake or their attitude or anything, just left me a message "your account is now up to date". Umm...it never wasn't!!!! And this is with a dentist I have a long history with, lots of work done the last 5 years, never a late payment. I am a GOOD patient for this practice. But their attitude on this incident definitely has me thinking of leaving.