The bug was discovered by Karam Daoud and it couldn't be any simpler.
All you have to do is dial a number from the emergency call screen and keep tapping the call button until you cause a crash which displays the Apple logo. Once that happens, your iPhone will call the number in the background and when the app returns the call duration will be displayed.
We've confirmed this works as described; however, we had to repeatedly tap the Call button for about 30 seconds.
This is the second security related bug that has been discovered in iOS 7 in past few days. Jose Rodriguez found the first bug which lets you bypass the lockscreen passcode to access all the photos on an iOS device and send emails and tweets.
Take a look at the video demonstration below...