That's how Inspector Gadget answered his phone. More seriously, people picking up the receiver of a Western Electric Model 500 would grip the receiver tightly;

That's how Inspector Gadget answered his phone. 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.