How do I recover my old iPhone passcode after syncing with a new iPhone?

After I synced my new iPhone, the old one reset and requested the passcode, but my old passcode didn't work. I saw various options, but I can't see "forgot passcode" on my screen to follow the steps. For now, its screen shows "iPhone Unavailable try again in 15 minutes," already more than 20 minutes.

[Re-Titled by Moderator]