The T-Mobile Galaxy S III firmware doesn't even allow access to the menu which allows the end user to select a particular voice codec. It's actually showing up as the codec selected by the phone for a call in the ServiceMode menu (accessible by entering *#0011#). This means that it is network-enabled.
Remember, this implementation of HD Voice does not require LTE, just UMTS. It also aligns with their strategy of being the "un-carrier," implementing features that other US-based carriers refuse to.