What should I do about knowing a young man and woman who are using obscene language over the phone; should I threaten or keep silent?

"A conversation with a foreign woman using indecent language is a reprehensible act that must be changed by hand, tongue, or heart. Whoever becomes aware of such a conversation should advise the doer, and if the advice is not effective, they may threaten to expose the matter so that the person is deterred. As for the girl, one should seek a righteous woman to remind her of Allah, otherwise, she should be threatened with exposure."