T-Mobile has been supporting mobile HDVoice for over a year. However, my sense is that not very many people are actually experiencing HDVoice. If they are, they might not even know it.
For example, two of my associates have the Google/LG Nexus 5 handsets on T-Mobile’s network. Both are the sort of people who would hear and appreciate the difference that HDVoice makes. That said, both were initially of the impression that the Nexus 5 did not support HDVoice on T-Mobile!
This gave rise to the idea that we should devise a simple way to verify that a call was in HDVoice. If convenient, this would allow anyone interested to make a call between two handsets and know with certainty that the devices and call path was actually delivering HDVoice.
Devising such a test turns out to be very easy in a world of smart phones. All you need is a tone generator or a recording of a specific continuous tone.