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Ok this is where Im at, I have had this happening since december of 2010 after update to 4.2.1. Connected to iTunes and restored, originally would get a 1013 error message and after couple of time finally got it back to working condition by deleting a line in a file in the system 32 folder. Then I started to get this, restore needed message and losing my IMEI and ICCID. Tried to downgrade to 4.1 and would get a 1015 error, then restored back to 4.2.1. Still get this error, and happen on average twice a day. Took it to apple genius, they told me it software related, and is corrupted. Also note, none of my 3rd party apps work, would close as soon as I try to open them. Well the guy at apple told me to restore again, but to restore as a new phone instead from back up. Done, and still am stuck with this situation with my baseband dropping out. Tonight Im going to try 4.3.1 and will see what happens.
3GS iOS 4 Modem firmware 05.14.04