How to unlock iPhone 4 baseband 4.12.01 on iOS 5.1?
If yours is locked to other carriers, you can use the steps here.
If Apple stops signing (making it available for download) a specific firmware version then you won’t be able to downgrade baseband 4.12.01 to lower version such as 1.59.00, 2.10, 3.10 or 4.10.01. Currently there’s no method
of unlocking or downgrading baseband 4.12.01.
The only possible alternative is to replace the modem firmware chips. See here.