For those wondering, it was a GEICO advert from 1995: https://www.youtube.com/watch?v=flP-o0ydkvo

("Don't cheat the phone company, save money the legal way.")

I think the GEICO one is the most posted on the Internet because I also remember the same conceit for one of the "direct access long distance" numbers (Sprint's 10-220) and for, I think, 1-800-COLLECT.