I’ve moved my iPhone 3G from AT&T to T-Mobile’s “Even More Plus” fully unlimited plan for $79.99 a month. I’m gaining unlimited minutes, but losing 3G and visual voice mail. I only had 3G enabled 5% of the time anyways, to try and save battery power.
After jail breaking the device with blackra1n, installing sn0w, and inserting my T-Mobile SIM card, these are the settings I have used:
Springboard (aka Home screen) > Settings > General > Network
Enable 3G – OFF
Data Roaming – ON
Cellular Data Network
MMS Proxy: (blank)
MMS Max Message Size: (blank)
MMS UA Prof URL: (blank)
After a reboot, everything worked great!