You can also ask someone to mime answering a phone call. Some people will extend their pinky and thumb, but younger generations will just hold their palm to their ear or position their hand like they're gripping a flat device.
More seriously, people picking up the receiver of a Western Electric Model 500 would grip the receiver tightly; extending your thumb and pinky from this grip gets you the "pretend phone" hand mime. Otherwise it just looks like you're holding your fist to your ear. People grip a cellphone quite differently, generally with their fingerpads or the middles of their fingers.
Or sexual depending on the angle and tightness of fist.
It is common to cartoons and TV shows so some people will have learned it from there, but I'd guess the mine predates those shows. Maybe it when predates TV being common in people's homes.