AT&T has launched an internal investigation into what caused some Hawai'i customers to experience dropped calls and difficulty making calls or receiving data on Tuesday.

The company said it was able to restore service to all Hawai'i customers at about 9:30 Tuesday night.

"Our priority was getting service back up and running," Courtney Pendleton, an AT&T spokeswoman in Southern California, said yesterday. AT&T also is looking into how many customers were affected by the service failure.

"Customers experienced intermittent coverage," Pendleton said. "Some of them could still make and receive calls, so it's hard to pinpoint how many were affected."

AT&T experienced an unspecified hardware problem Tuesday that caused some customers to lose service at times.

Pendleton said the problem seemed device-specific, with some types of phones having problems and others continuing to work.

People who work in Downtown Honolulu reported on Tuesday that they were having difficulty making calls with their iPhones, and that they didn't have any signal-strength "bars" to indicate that phone service was available.

Others reported sporadic service and also seeing enough bars on their phone to signify they could make calls, but said they could not connect with land-line telephones.