What can be done when you dial *#*#4636#*#*, the Rezound can be used on a gsm network or others?
You can switch to GSM via that menu if you have a GSM sim card. Voice and SMS are known to work however nothing else will. I believe this only works with the hybrid Sense 4 ROMs but that's research you should do before attempting.
Helpful Rezound Links:
Guide to Unlocking, Rooting, ROM'ing, and FAQ
Did you know: You can upgrade your Rezound to ICS without rooting?
Fun Rezound Fact: The antennas for the Rezound are in the battery cover.
Don't forget to backup your flash player install before they remove it from the Android Market.
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