The irony of this instructional diagram is it would be needed today for anyone who had never seen one before.  The rotary UI reached is primacy in the late 1970s, before push button phones took over. For anyone born today, and never saw it used in a movie, would have a hard time figuring it out.

The problem of course is the diagram fails to explain the most important thing: how to actually dial a number.

The design itself had the curious side effect of making 0′s the most expensive number to dial, as it took the most work and the most time.

(Source: unknown)