The city's new $2.1 billion system was built to handle 50,000 calls per hour, but crashed twice last week.
On June 4, first responders were delayed by more than four minutes responding to the car crash on the Upper West Side that killed Ariel Russo, 4, and injured her grandmother. Fire Commissioner Sal Cassano blamed the delay on human error.
Despite nearly six months of testing, the network went down again for 16 minutes the next day, forcing operators to write call information down on pen and paper.
Liu compared the problems to the city's $500 million employee time-keeping system CityTime.
"In this case, system failure would have far more disastrous consequences," Liu said. "How can it be that $2 billion has bought an E911 system that must resort to pen and paper? What happens if an extended heat wave taxes the system?”
Earlier Tuesday, police commissioner Raymond Kelly defended the system, arguing that the 911 system was in desperate need of an upgrade, and that "little bumps" and "hiccups" were to be expected as it got off the ground.
"This is a big, complex system."