With cars, the need for mobile communications rolled in. In the 1928 Detroit police cars were the first police cars to be equipped with radio. First the radio was only one way communication, allowing to "Calling all cars." The radio dramatically improved response times. Messages were sent over AM radio frequencies.
To report to the police station, the officer still had to go to a police box. Although the fire department used radio sometimes, the fire department mostly relied on its fire alarm boxes.