My old flip phone used the earphone as the antenna. The radio wouldn't work without it plugged in.

EDIT: It was a Nokia 6086, and the radio was an FM receiver.

https://www.gsmarena.com/nokia_6086-1798.php

https://www.phonescoop.com/phones/phone.php?p=1138

Anything that plugs into the head phone jack acts as the antenna.