A group of researchers from Ben-Gurion University, Israel, identified that it was possible to hack this system and disable it within an entire state for a prolonged time span through a simple Telephone Denial of Service or TDoS attack. The team believes that only 6,000 infected smartphones can do the trick in any given state while if there are 200,000 compromised phones then it would be possible to compromise 911 services in the entire USA. The tactics could be used for days with techniques that will prevent authorities from stopping the fake calls.
Source: https://www.hackread.com/911-can-be-hacked-tdos-attack/