This page of the Biomutant guide contains a walkthrough of the Find the Pling-Plong-Booth side quest. Here we've described everything you need to do in order to complete the quest and earn the rewards.
Starting the quest
How to start: Talk to a man in town - Flibberdit.
Reward: 100 XP.
Talk to the man in the tower. He will mention booths appearing in cities.
Location: Krossway 3G.
The Pling-Plong-Booth is red so it can attract your attention. All you have to do is go inside and adjust the dials accordingly - so that the colors match.
If there are enemies in the area, you must kill them first. Otherwise, the character will not start the quest.
Location: Krossway 7G.
The booth is red so you should easily spot it when you travel across the main road. Just jump off the mount and enter the phone booth.