Install ONLY if you:

- Unlocked your phone with AnySIM, iUnlocker or any old method - Have, or have previously had firmware v1.1.1 - Modem Firmware is 04.01.13_G - IMEI is starting with 0049xxx (unless you have v1.0.2)

Check this in Settings → General → About

After phone is fixed, you MUST restore your phone in iTunes by holding Shift/Option and click Restore and choose firmware manually (either 1.0.2 or 1.1.1).

Credits goes to Iphone Elite Team, Iphone Dev team, and Apple.