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How to jailbreak the iPhone 3G 4.2.1 with baseband 5.15.04 for use with a T-Mobile prepaid SIM.

Obtain T-Mobile SIM
Download the 4.2.1 IPSW Firmware (download)
Download and run redsn0w to jailbreak
– use above IPSW file when requested
– Options: Install Cydia (required), Install iPad baseband (required), Enable homescreen wallpaper (optional)
From Cydia, install UltraSn0w to unlock
Update MMS settings for T-Mobile

Phone should now be on the T-Mobile network with the ability to send text and MMS messages.

Import/recover old SIM contacts (optional)
– Install old/obsolete SIM card
– Go to Settings -> Mail, Contacts, Calendars -> Import SIM Contacts