Ex-wife's unpaid bills are resulting in garnishment of my wages

My husband's ex-wife defaulted on $900.00 worth of dental bills for the children. The decree states she pays for the first $61.00 and after that we pay our 36% portion of the remainder. She was served papers and accepted them on behalf of herself and my husband (he carries the insurance) and did not respond. We just received a garnishment letter as the dentist turned it over to a collection agency. We were never served with papers (we live three hours away), never received notification of a bill, and had no knowledge of this issue. What do we need to do? I have already spoken to the collection agent, and faxed a copy of the decree (they have been divorced over six years) and a release of writ of garnishment stating what was mentioned above. Do we need to file a motion with the court? Do we need to serve the ex with the papers, also? I am unsure what forms to use and how to stop the garnishment for a bill we did not incur.