beginning in 1938, a telephone bus (or van) was often supplied to reporters or the public, at events where many people would need to cover the news (Thank you Ed!)

This mobile telephone center has been put into service in Chicago in 1960. 

It was designed to assist reporters and officials at special events. The 32-foot bus carries 15 phones—two of them on the outside. It is also provided with 10 mobile radiotelephone channels, several hand-carried radio transmitters, and a powerful public-address system.

https://www.vintag.es/2022/11/telephones-on-wheels.html

On July 14, 1938, Howard Hughes and four companions landed triumphantly at Floyd Bennett Field in New York, setting a new world record for circumnavigating the world of 3 days, 19 hours, 14 minutes and 10 seconds.

in the days before cellphones one needed to find a landline if you wanted to brag to your friends about being there on that historic day. Enter the New York Telephone Company's "public telephone truck."

While the press had 47 of their own dedicated telephone lines to deliver on-the-spot news of the Hughes flight, members of the public could line up to use one of the five public payphones available in a truck parked near the airfield.

At that time, mobile public telephone trucks were only being used in New York and Maryland (probably Washington DC)

https://psmag.com/environment/wireless-public-telephones-on-wheels-46448

https://digitalarchive.tpl.ca/objects/201802/free-phones-mary-clair-harrold-of-laird-drive-uses-one-of-t