AT&T has increased by more than a third the calling capacity for its wireless network in Hawaii, hoping to avoid a recurrence of the statewide service problems its cellphone customers suffered in the wake of the March tsunami that hit Japan and then headed to the islands.

So much demand was placed on the local AT&T network as Hawaii braced for a possible tsunami the night of March 10 Hawaii time that many of the company’s customers, including some neighbor island officials coordinating emergency efforts, were unable to use their phones for hours.

Since March, AT&T has added two cell towers on Hawaii island, installed additional communications channels to many of its existing towers, improved its main network switch in Mililani and upgraded the “backhaul” connection between the towers and the regular telephone system, resulting in a 37 percent increase in calling capacity, said Dan Youmans, AT&T’s state president for external affairs in Hawaii and Washington state.

Asked if the upgrades would prevent AT&T’s network from being overwhelmed again during another major incident, Youmans said, “It’s a difficult question to answer. It’s hard for us to predict what the next natural disaster is going to look like.”

But the company’s network is better prepared, he added, and the lessons learned from March have helped with disaster planning here and elsewhere.

“For us, it was certainly a very good learning experience,” Youmans said.

Almost immediately after the devastating tsunami waves struck Japan on March 11 and a tsunami warning was issued for Hawaii, traffic on the local AT&T network surged to a level nearly 25 percent more than those on the busiest days of the year, he said.

That meant that many customers were unable to access the network. He compared the situation to everyone in Hawaii trying to get onto the H-1 Freeway at the same time to drive to the airport, causing major traffic jams.

AT&T customers throughout the state were affected, including on Maui and Hawaii island, where some county personnel involved in emergency management lost their cell service.

“It got progressively worse through the evening,” said Quince Mento, Civil Defense administrator for Hawaii island, where AT&T is the predominant wireless carrier for the county government.

“The impact was pretty tremendous to us,” added Keith Regan, Maui county’s managing director, who said some key managers lost cell service for much of the night.

Once service went down, many Maui and Hawaii island personnel relied on walkie-talkies, landline phones and other means to communicate, enabling them to continue doing their jobs preparing for a possible tsunami, the two officials said. Both said the outages did not cause any major safety problems.

“It just made things more difficult,” Regan said.

Both officials said they were satisfied with the measures taken by AT&T to upgrade its network.

But the real test, Mento added, will be when the next major event happens.

“We’ll just have to wait and see how it evolves,” he said.

A city spokeswoman said no phone problems similar to what the neighbor islands experienced were reported by emergency responders on Oahu. The main responders did not have AT&T phones.

The spotty service, however, affected the company’s customers throughout Oahu. One customer started a Facebook page to lament the problems of that night.

Youmans said AT&T is continuing to make improvements to its system, including plans to add two more Hawaii island towers and a Kauai tower this year.

Youmans would not disclose how much AT&T spent on the upgrades since March or how many towers the company now has in Hawaii, saying that information is proprietary. Between 2008 and 2010, AT&T has invested $90 million in its Hawaii network, according to Youmans.

He said some improvements made since March already were in the works before the tsunami, while others resulted from the March problems. The improvements have added data capacity to AT&T’s network as well.

The surge that reached Hawaii caused about $30 million in damage, the state has estimated.