Yet another great article by LifeHacker.
Here’s how to set it up if you aren’t currently using a passcode:
- Unlock your iPhone and open Settings > General > Passcode Lock.
- Toggle Simple Passcode to Off.
- Tap Turn Passcode On (assuming you don’t already have it turned on) and enter your new passcode using only numbers. You’ll see the standard alphanumeric keyboard during your initial passcode creation and confirmation, but don’t worry—if you stick with numbers, you’ll get the numeric keyboard later.
Click this link for the full post.