Here's a passage from Stephen King's Night Surf short story:
The Massachusetts station was better, but we could only get it at night. It was a bunch of kids. I guess they took over the transmitting facilities of WRKO or WBZ after everybody left or died. They only gave gag call letters, like WDOPE or KUNT or WA6 or stuff like that. Really funny, you know - you could die laughing.
The sentence "They only gave gag call letters, like WDOPE or KUNT or WA6 or stuff like that" made me puzzled. Does it refer to some kind of telephone jokes, prank calls? What is it about? What are gag call letters and what's the humor in the WDOPE, KUNT and WA6 acronyms to make the radio listeners laugh?