Want to prank your buddies by changing voice on phone? try VoiceChanger for iPhone. VoiceChanger is a new Cydia tweak that allows users to change their voice during a call. There are multiple effects available in VoiceChanger, like Dark Lord, Tenoro, Smurf and Rumplestiltskin.
VoiceChanger has to be configured before using it, later it adds to the stock Settings app. It only works with iPhone 4S and iPhone 5. You can purchase VoiceChanger in Cydia store for $2.99