Extensions of telephone distribution lines made after June 5, 1968, which are necessary to furnish permanent telephone service to new residential buildings and mobile homes, shall be made underground. The extension of service shall be made by the utility in accordance with the provisions in these regulations. These regulations apply to: Extensions of telephone distribution lines in subdivisions where distribution cables and service lines will be placed underground to provide permanent telephone service to any number of residential buildings or mobile homes; and An extension to provide permanent telephone service to a single residential building or to a mobile home.