Called my wife yesterday, (both use iPhone 5's on T-Mo) and experienced HD Voice for the first time. Was pretty neat. Sound was much "clearer" but background noise was awful. She was inside, myself in the car and I could hear the neighboring conversation someone was having on her end, in a quiet bridal boutique. BTW my wife hated it. Said it was too weird and doesn't like that it can't be turned off. I loved it. So T-Mobile's HD voice isn't exclusive to LTE, but works well on their "4G" as well. BTW these phones are AT&T phones using the 1900mhz AWS spectrum for speed. So there ya go. Anyone else able to try out the HD Voice? Must be on two HD Voice capable devices, on at least 4G and on the same network.