Sony Xperia ST21i forgotten PIN help

I set a PIN screen lock on my Sony ST21i (Android 4.0) and forgot the code. No encryption, no root, and the phone won't show up a "Forgot PIN?" button. How to regain access to my phone? There is no important information in it, so any solution that deletes all data is fine. Help!

Edit: Didn't work:

• Android Device Manager


• Trying to boot into "recovery mode" using various key combinations found online