Cap'n Crunch used to include a prize whistle that unintentionally allowed people bypass international calling fees.
If you’re old enough, maybe you’ll remember this one yourself. Back in the early seventies, Cap’n Crunch came with a cleverly named “Cap’n Crunch Whistle” as a prize inside. The sound the whistle makes (which you can hear at the source) is very similar to a 2600 Hz tone. That tone just happened to be the same tone involved in long distance phone calls. A 2600 Hz tone told the At&T long lines that, basically, the long distance call had been authorized and added to the customer’s bill.
Somehow someone discovered the similarity between the tone on the phone and that on the whistle. The whistle then, when blown into the phone, would sidestep the billing system (back then, long distance calls were expensive). Obviously, the whistle didn’t last long. But before you run out and try to buy one on eBay, you might want to know for many years now long distance calls have been handled by digital phone switching, so this no longer would work.