Larry knew when his grandmother was calling because her ringtone was set to the theme song from "Shaft".
An example of a ringtone is the sound like a classic telephone that you set to play on your cellphone when your grandmother calls.
ringtone - Computer Definition
The audible sound made by a telephone to announce that a call is coming in. The traditional ringtone was in the 440-480 Hz range, but as cellphone usage grew, it became obvious that ringtone differentiation would become important. Modern cellphones support a wide frequency range that allows for several bars of music to be played. Phones come with a selection of built-in ringtones and accept new ones from one or more ringtone services that are downloaded for a fee. Ringtones have become a fad, providing another mode of self-expression. MP3 has become popular as a ringtone format because it supports voice. MIDI and Nokia's RTTTL (Ringing Tones Text Transfer Language) formats are used, but for music only. Depending on the phone and format, ringtones can be sent via text messages (SMS), but there is no standard for ringtone transfer for all phones and services. See moantone and Mosquitotone.