Apparently nothing. From everything I've read from credible sources, this is an internet myth. It was apparently started by well-meaning people trying to warn others from who knows what.
A voice recording of someone saying "yes" isn't a legally binding contract.
Urban legend notwithstanding, the Better Business Bureau, Federal Trade Commission, and Consumer Federation of America confirm there have been zero records of anyone being defrauded by a "yes" phone recording.
Look it up for yourself. I did years ago, then again this morning. The answer remains that this is nothing more than an urban legend. It continues to be amplified by blogs, social media and (lazy) news outlet that pick up the legend, but please note, they never cite specific cases because there are none.