The internet and social media have been rife with comments from iPhone users challenging others to say '17' to Siri, and then see what happens.
Much like a similar controversy a few years ago, with people encouraged to say '108' for fun, neither numbers are something you should speak for the hell of it.
Why? Because your iPhone will immediately call the local emergency services.
Most countries around the world have different numbers to call if you get into trouble. But knowing which number to dial in your area if you're travelling abroad is something most of us don't consider when packing our cases for a trip away.
To help tackle this, the Apple developers and programmed your iPhone to automatically call the local emergency number, even if you say the code for your home country.
So saying '911' in France will get you immediate help in France. Likewise, speaking '17' will do the same in the U.S. Because it is an emergency number in many other countries.
The feature has been used for jokes and playing pranks on people since its introduction. But we don't recommend phoning the emergency services unless it's vital, and hope it's something you'll never have to use.