A new jailbreak app to unlock your iPhone’s homescreen using the volume buttons is VUnlock.It is available on cydia for free.
If you don’t know how to install VUnlock on your jailbroken iPhone i’m giving you the instructions:
* 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.