YSK some smartphones would reboot upon trying to call to 911 emergency services. Here's how to properly test yours.

Many people would just dial 911 and immediatelly hang up, which is about the worst thing you could do - the police will try to locate you to make sure everything is okay, instead of dealing with actual emergencies that might be happening. This can be avoided by first calling your local police department to let them know what you're doing. Afterwards you can try calling 911 from every smartphone in your family to make sure they won't fail you when you need them the most.

Source.