“CSI Cyber” on January 31, on CBS, aired a particularly disturbing episode “Going Viral”, where a psychopath or domestic terrorist infects that 911 system (in NYC) with malware to prevent calls from going through. Then the criminal stalks a particular victim and “gets off” on watching the victim’s fear.
But whether this is even possible would be a good question. The 911 system, like the power grid, should not be accessible from the public Internet, but it might be infected by an inside job with a thumb drive or some sort of external device, or it might be physically attacked somewhere with a device. The major carriers (Comcast, ATT, etc) probably do manage the infrastructure of the emergency systems, so it is conceivable there is some point of potential compromise.
Back in 1998, author Edmund Contoski had suggested the idea of a telephone virus in his novel "The Trojan Project" (American Liberty Publishers, Minneapolis).
Ted Danson and Patricia Arquette star. Danson’s charcter (D.B.) meets a woman who sent him a mistaken text. I have gotten these, maybe once or twice a year.