How to get Refund of over payment made to dental office

Me and my husband have cigna hmo dental insurance. We were made to pay an upfront cost for an appointment at dental office in may last year. Dental office belongs to a chain (US). They said their system does not show the services were covered and they will refund if it does, at a later time and if we dont, it will get sent to collections. We went ahead and paid. Cigna confirmed they are covered, made multiple calls to office/voice mails – no response. I went to the office and asked about this and gave all details. They said cigna should refund, not them and they will call cigna and look into it – no response.

Cigna suggested to create an appeal and processed it last october. Sent letters to me that dental office should refund the overpayment. And that they sent same letters to the office as well.

We were out of state for couple of months. Returned in Jan. Called office 2 weeks ago – was told the manager will get back – left voice mails – no response.

I am no longer using this office’s services.

How should we approach this problem? Thinking to drop by their office this week. How to get a solid response from them and not get sent back like last time?

Thanks in advance!