The modem David Lightman used to connect his computer to the outside world, as seen here and in the movie, was an IMSAI 212A. In reality though, it was only labeled that way for the movie. IMSAI never actually produced or sold such a modem. What was actually used in the movie was a Cermetek 212A modem, which could communicate at 1200 (!) baud.

Back in the day before computer modems could be directly connected to a phone line, phone handsets were placed in acoustic couplers connected to a computer, in order to convert sounds to electrical signals (and vice versa). In WarGames, an acoustic coupler is used prominently by David Lightman. Ironically, though, it wasn’t actually needed since his ISMAI (er, Cermetek) 212A modem didn’t need a coupler. It was included purely for the visual effect. Ahh, movie magic!