The cell-phone providers falsely advertised the security of their systems, according to the Los Angeles District Attorney's Office. During an investigation, "cell phones purchased by undercover investigators were easily hacked into, enabling the voicemail to be changed at will," the district attorney said in a statement Thursday.

"Hacking into voicemail allowed messages to be changed or erased. Important information could be removed from the voicemail and phony information could be inserted," the district attorney said. "Imagine the havoc that could result."

Investigators were able to hack into voicemail accounts using something called a SpoofCard. SpoofCard's software lets people display any number they want on caller ID and has been used to access voicemail systems that do not require passwords such as those used by Cingular (now part of AT&T) and T-Mobile.

In a statement, T-Mobile said that customers who want to add password protection to their voicemail should call voicemail, then press the star key to interrupt the greeting, and then press 5 to be prompted to change their password.

"T-Mobile cooperated with the district attorney and is pleased to have reached resolution on the issue," the company said.