New York City’s aging 911 dispatch system, which has recently faced problems, went offline a number of times on Monday, for a total of 90 minutes, causing delays and forcing operators to use pen and paper to record emergency calls, the authorities said.

The failures, at least six in all, slowed 911 response times somewhat, said Francis X. Gribbon, a Fire Department spokesman. By Monday evening, the system was fully operational again. It was not yet clear what caused the failures, Mr. Gribbon said, though he noted that the problem seemed to have occurred in the system’s data storage equipment.

No calls were lost, and two other systems used to dispatch police officers and firefighters remained online, the Fire Department said in a statement.

The 911 dispatch system has encountered trouble recently, as the city works on a $2 billion upgrade. Spikes in the number of calls have occasionally overwhelmed the system, causing disruptions. Technical failures and human error have sometimes resulted in costly delays in emergency response times.