The hackers ported the Daly City woman's landline number to a cell phone, then used it with other stolen information to steal $25,000 from her bank account.
Copper thieves are targeting AT&T customers and affecting landline service in Baldwin Hills and the company is now trying to solve the issue with the help of law enforcement.
AT&T wireless service was down for some customers for parts of Tuesday because of a software issue that has since been resolved, the company said.
AT&T blamed an "illegal download" on a third-party cloud platform that it learned about in April - just as the company was grappling with an unrelated major data leak.
AT&T has resolved the "interoperability issue between carriers" customers were experiencing on Tuesday, the company said in a statement.
A landline phone may be a relic of our days before cellphones, but for thousands of AT&T customers they are still a daily part of life.