French kids started doing this in the 1930s with the talking clock (a French invention BTW). The clock was basically a busbar: callers were all connected to the clock. When it was not speaking the time (at the minute) callers could speak and be heard by others. And it was a free call!

The phone system had switched to digital by the time I started using it so I never got to experience this, but in exchange I could use Minitel!