From the introduction:
The number you have dialed is not of this earth. You have opened the gateway to an alternate reality. You are being connected to another world in another time, where you are another person. Welcome to the world of FIST!
F.I.S.T. offered several adventure quests, in the style of Fighting Fantasy Gamebooks, which were played over the telephone. One of the main attractions of F.I.S.T. was the presence of pre-recorded sounds applicable to the action, somewhat like a radio script, behind the voice of the narrator. Participants called a listed telephone number, made some setup decisions, and then listened to an introductory piece of the adventure. At a given point they were offered a decision and used a touch-tone telephone to select a response. The response would then initiate the next stage of the adventure.
F.I.S.T. play incurred a per-minute charge to the calling number, including any applicable long-distance costs. The game was available only within certain calling areas in the UK and the USA.