"When a customer enters their username and password to an online banking site, PhoneFactor intercepts the signal and sends an encrypted message to the PhoneFactor voice response unit. The customers cell phone rings, and they?re asked to push a button or enter a code if they?d like to login. If not, some configurations allow users to immediately report fraud.

http://www.americanbanker.com/btn_article.html?id=200805283ULS479A&pagenum=1&numpages=5&showallpages=true
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