The FBI has warned that telephony denial-of-service attacks are taking aim at emergency dispatch centers, which could make it impossible to call for police, fire or ambulance services. A TDoS attack is designed to prevent incoming and outgoing calls, by flooding a target with junk calls. The FBI also warned that TDoS attacks could be used in conjunction with a physical attack, when calls to 911 and other emergency numbers would crest. The resulting increase in time for emergency services to respond may have dire consequences, including loss of life.
Source: https://threatpost.com/tdos-attacks-emergency-first-responder/164176/