An issue was discovered in certain Apple products. iOS before 10.3 is affected. The issue involves the "Quick Look" component. It allows remote attackers to trigger telephone calls to arbitrary numbers via a tel: URL in a PDF document, as exploited in the wild in October 2016.
https://www.engadget.com/2017/03/31/apple-fixes-ios-loophole-911-overload/
https://support.apple.com/HT207617