When Sheldon interrupts Georgie's phone call with his modem, the modem immediately starts making the squealing sound that modems make to establish connection. It should've first dialed some phone number; Georgie should've heard a 7-tone sequence through his ear piece.
Modems, depending on how fast they are and their connection, make different noises when connecting. The noises made when Sheldon connects both at home and at the school library were for modem connections of 28.8k or higher, which would not exist in 1991. (28.8k modems came out in 1994.)