I don't know about NYC's history.
Phila's old plain 911 system sometimes had busy signals or no answer on busy Saturday nights.
Phila had problems with the 911 _operators_ messing up. One time desperate pleas for help were not properly prioritized and a kid being attacked died as a result. The parents told the city they wouldn't sue if the city would use the money to fix up the system, which it did.
The murderers got only 5 years in prison thanks to the courts. That was a disgrace.