Does "reset network settings" on iPhone enable me to change mobile carriers? Say, for example, if I have the iPhone 4S, and I select "reset network settings" in "General", would that enable me to change mobile carriers from AT&amp;T to T-mobile?
To change carriers you would need to talk to your carrier, if your phone is not locked all you need to do is change the sim card and then activate it.

I've just changed to AT&amp;T in the US from an Australian carrier and the process was very simple.