Apple recently rejected camera+ app that has ability to take photograph using volume buttons, after that another app with same ability appears on Cydia now there is another app called Vunlock that is using volume button for another purpose and this time it is used to unlock the phone if you like this idea you can grab it from Cydia and it is free.
You can follow these instructions to install Vunlock on your jailbroken iPhone:
Launch Cydia from your jailbroken iPhone’s homescreen.
Tap on the Search tab and search for Vunlock.
Tap on Vunlock and then on the ‘Install’ button to install it on your iPhone.
That’s it. You should now be able to use the volume buttons to unlock iPhone’s screen.