As far as I understand, it's not. Check www.htc.com/www and look at the One S specifications.
Doesn't support 1700 AWS bands nor does it have the 1900 mHz (AT&T 3G, soon to be T-Mobile HSPA+ as well).
A software hack won't fix bands that don't exist in the hardware.
The 850 mHz band is also AT&T 3G, but it is not as widespread around the country as the 1900 mHz band. So, you're getting AT&T 3G on 850 mHz because that is available on the One S.
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