SAN FRANCISCO (AP) _ The use of an extension telephone to listen in secretly on someone else's conversation is a violation of state privacy laws, which allow victims to sue for $3,000, the California Supreme Court has ruled.

The high court ruled Monday in favor of Richard Ribas, who contended a friend of his ex-wife listened to a phone conversation between the two without his knowledge and then testified about the conversation at a divorce arbitration hearing.

Justice Stanley Mosk said the ruling does not mean, however, that children can sue their parents for listening in on their conversations. State law protects parents from lawsuits ''when they reasonably and prudently exercise their disciplinary authority,'' he said.

The suit was filed in Santa Clara County Superior Court by Ribas after he said his ex-wife, Alice Laughlin, called from the phone of a friend, Joan Clark. Ms. Laughlin later sued her husband to set aside the divorce, and Ribas said Ms. Clark testified at an arbitration hearing about the phone conversation she overheard.

He sued Ms. Clark for invasion of privacy and emotional distress, citing the state law that makes it a crime to wiretap a conversation or attempt to ''learn the contents of any message, report or communication while the same is in transit.''

Superior Court Judge Bruce Allen ruled that the law did not apply to the use of phone extensions and was upheld by the 1st District Court of Appeal. But the Supreme Court ruled unanimously that the law did apply, dividing only on the question of whether phone company authorization was a valid defense.

The privacy law prohibits ''far more than illicit wiretapping,'' Mosk said in the majority opinion.

In this case, Mosk said, Ribas was not entitled to damages for Ms. Clark's testimony about his conversation because state law bars suits over statements at judicial proceedings. But the privacy law allows victims to sue for $3,000 without proof of actual damages, Mosk said.