The Philippine Criminal Investigation and Detection Group said it worked with the FBI to arrest the suspects last week. The hackers, according to investigators, worked for a group that helped finance a deadly 2008 terrorist attack in Mumbai, India.
AT&T's systems weren't compromised but some of its customers were targeted, spokeswoman Jan Rasmussen said Saturday. AT&T cooperated in the investigation with the FBI, she said.
The FBI would not comment Saturday.
AT&T wrote off some fraudulent charges on customers' bills, but Rasmussen wouldn't say how much. Philippine police put the alleged hacking cost to AT&T at $2 million.
Last Tuesday, AT&T said that hackers unsuccessfully attempted to link mobile numbers with online customer accounts, but it wouldn't say on Saturday if that incident was linked to the arrests of the four people in the Philippines.
The hackers were working on commission for a terrorist group linked to Muhammad Zamir, according to the Philippine police. Zamir, a Pakistani, was arrested in Italy in 2007, where he was running a call center that collected money from callers but then routed the calls through hacked phone lines. He also allegedly sold international access codes for long distance calls that were gathered by Filipino hackers.
Since then, police said, Zamir's group has been taken over by a Saudi national. Philippine police didn't name the group, but India has blamed Lashkar-e-Taiba, a Pakistan-based militant organization, for the Mumbai attacks. Three years ago Saturday, 10 Pakistan-based gunmen laid siege to India's financial hub, killing 166 people.