After Hurricane Sandy put entire cities and towns in the dark, people across the East Coast immediately discovered the downside of the emerging technology age: It has become harder to communicate in emergencies.

Over the past decade, consumers have rushed to replace their landlines with cellphones, digital phones and other sleek gadgets. The problem, some analysts say, is that these devices require electricity. So as the power goes, so goes the ability to communicate with friends, family and emergency responders.

From the perspective of these analysts, the digital portrait of America is not encouraging.

About 36 percent of American homes are wireless. Pay phones dwindled to 550,000 in 2009, down from 2.1 million a decade earlier, according to the Federal Communications Commission.

These traditional methods of communication are among the most reliable in an emergency, experts say. Landlines can work even when the electricity is out.

“Copper-wire phone networks were built with stability and dependability in mind, because they were public utilities and were supposed to withstand hurricanes and floods,” said Harold Feld, a senior vice president at public interest group Public Knowledge.

Critics of the telecommunications industry say it is not doing enough to ensure that Americans can communicate when they need to the most.

AT&T, Verizon and other companies have been making a push to invest less in wireline networks, as their popularity has dwindled. They and providers of Internet service pressed the FCC to repurpose an $8 billion fund, originally intended to bring traditional phone lines to rural areas, to finance broadband Internet instead.

Public interest groups have argued that wireless providers should provide backup power for each cell tower to ensure reliable service. But the industry has said that would be too expensive. Instead, they have said they would respond to emergencies by bringing in generators and extra cell towers only when needed.

But Sandy exposed a problem in this plan. The generators need gasoline — a scarce resource in stricken areas of New York and New Jersey because the gas stations didn’t have power to pump fuel.

Overall, in storm-affected regions, 25 percent of cell towers went down Tuesday as the storm hit. About 15 percent remained out Friday morning. Downed trees and flooding kept crews from repairing many towers, companies said.

Wireless carriers said they had made progress. Verizon Wireless, whose downtown Manhattan offices were flooded, said nearly 97 percent of its East Coast customers were able to resume at least some service. T-Mobile and AT&T this week agreed to allow free roaming on each other's networks in parts of the Northeast to handle the surge of calls.

“While wireless service has yet to return to normal levels, coverage is good,” Verizon said in a statement Friday afternoon. “However, telecom and residual flooding issues continue to be a factor.”

Verizon Communications said its business customers wouldn’t get their FiOs Internet service fully restored for about a week.

Fixed-wire phones, though, are not invulnerable. In Lower Manhattan, flooding cut off traditional phone lines earlier in the week. Pay phones suddenly came back into vogue in parts of the city.

“There will always be disasters that are damaging to any form of communications,” said Brian Fontes, chief executive of the National Emergency Number Association, a trade group that advocates for emergency-responder services. “It’s best not to rely on any one service.”