When the AT&T central office on South Stewart Street was swamped with flood water and 911 emergency service was knocked out countywide on Wednesday, Sonora Police Department Records Technician Debbie Dills said dispatchers received “phantom 911 calls.”

“They were ringing,” she said. “What was ringing, I don’t know.”

Police Chief Turu VanderWiel estimated that for 15 minutes the station could not receive any 911 calls.

Dispatchers who picked up the phone heard only static. The phantom calls were either a telecommunications malfunction, or there could have been a person on the other end of the line, VanderWiel said.

“If there were any legitimate calls coming in, they wouldn’t have been received,” he said. “No one has made it known to us that they couldn’t make it through, but we do expect there were some problems and some failed attempts.”

During the high point of the ice storm and flood, more than 30 calls in a little more than an hour came in. Then, between 3 p.m. and 3:30 p.m., the calls stopped completely, Dills said

“I think it was something that couldn’t be foreseen,” she said.

The dispatchers attempted to make outgoing calls and confirmed the lines were dead. They contacted the Tuolumne County Sheriff’s Office with a cell phone and asked them to flip the emergency switch, which reroutes the Sonora 911 calls to the county dispatch center.

Sgt. Mark Kerzich at the Tuolumne County Sheriff’s Office said the county dispatch took a few Sonora calls, but then their system went down as well.

“Once we were learned of the problem we were able to reroute the calls to the Calaveras County Sheriff’s Office and also to Merced CHP,” Kerzich said.

Until about 10 p.m., the Calaveras dispatch center used a computer-aided system to route calls for service electronically back to Sonora Police and the Tuolumne Sheriff.

The Sonora and Tuolumne dispatchers communicated with law enforcement digitally and with cell phones, and provided a list of cell phone numbers on social media to act as alternative emergency lines.

But all throughout the outage, Dills and dispatchers at the Tuolumne County Sheriff’s Office were in contact with representatives of AT&T to ask them: what went wrong? And when would it be fixed?

Ryan Oliver, a spokesperson with AT&T Media Relations, said underground cables connected to the AT&T central office on South Stewart Street were inundated by the flood and caused a service outage for a small number of Sonora customers.

The Sonora central office acts as a network hub for phone calls as they are routed to their intended destinations.

Dills said at 7:40 p.m., an AT&T representative notified her maintenance workers were drying the cables. Within a few hours, the non-emergency line was restored. Direct 911 service was reactivated some time during the night, she said.

Chuck Berdan, a retired dispatch manager for Alameda County Regional Communications Center and a former member of the state 911 Advisory Board under Gov. Jerry Brown, described the outage as a failure of the telecommunications infrastructure.

“It certainly is an old system. This goes back to the invention of the telephone and switchboards with an operator,” he said. “All of it has been modernized to where it’s computers and so on now. But if you lose the building itself, if there’s no backup, if there’s no other central office that feeds that 911 center, then nothing is going to work.”

Oliver said there were “redundancies at any central office” which mitigated a potential outage for all the Sonora customers who relied on the South Stewart Street facility. He said he did not know how many people relied on the center or why the law enforcement dispatch centers were disconnected while other users were not.

Berdan said a more reliable alternative to the cable and analog system was internet protocol (IP) technology, which uses computers to route calls electronically.

“The difference is between a car you buy in 1960 and the car you buy in 2018,” he said.

The IP technology would be less susceptible to weather-related catastrophes, he said, but it is stalled by state and national funding.

VanderWiel and Dills said they did not see the outage as an infrastructure crisis. Instead, they said it provided the department with an opportunity to develop alternative emergency backup plans such as a designated cell phone that 911 calls could be rerouted to.

“This was a big event for us, but fortunately it wasn’t a major disaster,” VanderWiel said.

VanderWiel said the only other emergency outage he knew of was from October 2014, when suspected vandals cut a fiber-optic line near Sonora and affected the dispatch centers for the department and the Tuolumne County Sheriff’s Office.

On Thursday, more than a dozen uniformed AT&T workers removed a mountain of muddy debris from the back parking lot of the facility and transferred equipment inside. Later, a small group of workers with AT&T and PG&E pumped water out from a manhole on South Stewart Street and William Street.

Berdan was not familiar with the infrastructure of the Sonora central office, but he said a lack of adequate backup could have precipitated the law enforcement outage.

“Typically, if one central office goes down, other central offices around it will pick up the load. If one phone line gets broken, then hopefully it isn’t the only one,” he said.

The cause would be more widespread since most calls processed through the facility likely didn’t come from landlines.

Eighty percent of calls were now made by cell phones following the precipitous drop of wired telephone use in the past 10 years, he said. 911 calls generated from cellular devices would also have to go through the facility after communicating with a tower before being transferred via microwaves or copper wire.

“At some point, it’s all going to use that central office to get to that dispatch center,” he said.

Contact Giuseppe Ricapito at (209) 588-4526 or gricapito@uniondemocrat.com . Follow him on Twitter @gsepinsonora.