Method for admitting new voice calls to a VoIP network. The folded fin includes a plurality of parallel fins and a plurality of upper and lower tabs interconnecting the fins. The part of the side surface in which the coating film has been removed or reduced by laser irradiation has a width of at least 5 mm from a front surface side toward a rear surface side of the building material, or extends over the entire side surface from the front surface side toward the rear surface side of the building material, or is formed more than a part in which coating film is formed.