"When your device makes this automatic call, it plays a looped audio message to emergency responders and out loud over your device speakers. This message informs emergency services that your Apple device detected a severe car crash and that you’re unresponsive. It also shares your estimated latitude and longitude coordinates with a search radius."
Use Crash Detection on iPhone or Apple Watch to call for help in an accident – Apple Support (UK)
If your iPhone or Apple Watch detects a severe car crash, your device can help connect you to emergency services.support.apple.com
Also 911 never assumes a call is a troll. They always send police.