How do you unlock your iPhone if you forgot the passcode?

Choose Settings > Phone > SIM PIN and turn the SIM PIN On. You will be prompted to enter the current PIN (iphone ships with a default SIM PIN, please see the table below to determine what should be entered). You can then tap Change PIN to assign a different PIN number. Note that the SIM PIN is separate from the Passcode Lock (Settings > General > Passcode Lock) which comes up when waking from sleep and turning on.