If you started a call with your cellphone close to a loudspeaker you could hear the connection being initiated through the speaker. Something like a tat-ara-tat sound.
IIRC transmission power was higher back in the day and the shielding of electronics wasn’t quite as good. So if you put your phone by a speaker the RF interference would be enough to be able to hear over the speaker.
GTA IV uses this sound when you're about to receive a call while driving. I remember being confused (and subsequently blown away by the attention to detail). I'm a little nostalgic for the sound so I'm glad it's preserved in that game.