During Thursday's blackout that darkened parts of the Northeast, most mobile phone subscribers were unable to make calls when they reached for their handsets. Only the truly persistent were able to get through after the lights went out, the largest US carrier, Verizon Wireless, acknowledged in a statement Friday.
In addition, the top six cellphone carriers in the United States said they were still battling outages on Friday morning.
The carriers blamed the ongoing outages on their cellular transmitter stations, which handle cellphone call traffic.
"We need electricity to power our cell sites, but when you don't have that, it's out of our control," Nextel Communications spokesman Chris Grandis said.
The transmitter stations are powered by electricity, and most have battery backups that provide three to six hours of additional operation. But when the blackout stretched beyond six hours, the stations that were still working went dead.
This is the second time a significant number of American cell phone subscribers had difficulty making calls during a time of crisis. The first was after the 11 September 11, 2001, terrorist attacks, when damage to the cellphone antennas on the World Trade Center towers and heavy traffic brought wireless dialling to a standstill.
Thursday's blackout caused a similar surge in mobile phone calls, creating about four times the usual traffic in the area, according to Verizon Wireless.
IDC analyst Keith Waryas believes America's 146 million wireless subscribers will continue to have problems getting calls through during emergency times, although he believes the 2003 blackout will force carriers to better arm their networks against electricity failures.
"These kinds of lessons are pointing to one thing: We expect our cell phones to always be on," he said. "This is a learning curve."
Since Thursday, many wireless subscribers in the Northeast have returned to the landline telephones they've increasingly given up in favour of cell phones. Verizon Communications, a part-owner of Verizon Wireless, said Friday that use of landlines was 300 percent above normal levels following the blackout. Traditional lines continued working, because phone companies have added back-up power throughout their systems, which also send electrical current to phones.
The six leading US cellular companies were still working to return to full capacity on Friday, in the face of ongoing problems.
"We're seeing strong improvements as power is coming back up," AT&T Wireless spokesman Ritch Blasi said. "But even with that, some calls aren't going through."
He added: "Our back-up systems worked as designed, but the extent and duration of this massive outage, which has been called the biggest ever, affected our network."
Among the worst-affected places Friday were Detroit and Bloomfield, Michigan. Some 75 percent of the Cingular Wireless network in those cities wasn't working on Friday morning, the carrier said on Friday.
Nextel's Grandis said the company was sending generators to power sites in the Detroit area.
Sprint PCS spokesman Dan Wilinsky said all of Sprint's 18.8 million customers should have service returned by Friday. But "multiple sites in the Northeast corridor continue to be impacted," he added.
Verizon Wireless reported a healthier outlook. "Throughout the country, service this morning is normal, except for a few isolated areas in the five boroughs of New York City, Detroit and Cleveland," the carrier said in a statement Friday.
