Hyphens, En Dashes, Em Dashes

Q. When referring to a telephone call for which the calling party is not charged, is it (a) a “toll free call,” (b) a “tollfree call,” or (c) a “toll-free call.” My own preference is for c first, then b. However, our marketing bunch uses a. Your thoughts would be greatly appreciated. This is driving me nuts (or at least nuttier than usual).