Use an attorney, there is no other viable option. Consider 10-30% of your 12k as a collection fee and pay the attorney. At best they respond to a single lette

Use an attorney, there is no other viable option. Consider 10-30% of your 12k as a collection fee and pay the attorney. At best they respond to a single letter from the attorney and pay, at worse you wind up paying about 40% of the money owed to the attorney. If the attorney asks for more than that find someone else.