Dental Office Threatening Me With Collections, Won’t Return Calls

South GA. Hopefully this is an appropriate sub for this question. Several years ago I took my son to a local dentist for routine exam, X-rays, and cleaning. He was covered by insurance. Now (several years later) they keep sending me statements saying I owe them over $300, and threatening to send me to collections. I have called their billing department numerous times. Usually there is no answer so I leave a voicemail and no one ever returns my call. I have been able to reach someone a couple of times, but they state the balance is coming from something in their “old system” and that they can’t see it and they’ll look into it further and call me back. They never call back. I have no issue paying a bill that I legitimately owe. However, I do not know where this balance is coming from, no one can seem to figure it out, and I am sick of receiving these statements. Is this worth taking to a lawyer? I don’t know what to do. I don’t really have $300 just lying around to send them if no one even knows what for.