N.C. Gen. Stat. § 44-51.2

No lien created by G.S. 44-51.1 shall be valid but from the time of filing in the office of the clerk of superior court a statement containing the name and address of the person against whom the lien is claimed, the name of the county or municipality claiming the lien, the amount of the unpaid charge for ambulance service, and the date and place of furnishing ambulance service for which charges are asserted and the lien claimed. No lien under this Article shall be valid unless filed in accordance with this section within 90 days of the date of the furnishing the ambulance service.