T-Mobile just announced its very own iPhone 5 and no-contract plans. One of the little subplots was that this would be the first iPhone to support HD Voice. We just went ears-on with to see if it's a difference you can actually hear.
Calling from one T-Mobile iPhone to another, my caller's voice came through loud and clear. You know how sometimes cell-to-cell voice calls can sound a little fragmented? Like little bits of sounds are being dropped? This didn't have that. It was a much more natural sounded call. It was a bit richer, and definitely fuller. I wouldn't say it was like night and day, but it's definitely a noticeable improvement. I then used the same iPhone to call an HTC One on AT&T and it sounded, y'know, like a typical cell call. It was very slightly broken up, and definitely didn't have as much dynamic range.
It's important to note that this isn't brand new technology; T-Mobile's iPhone 5 HD Voice sounds just like Sprint's Evo 4G LTE HD Voice from last year. But it's definitely the clearest tone you'll find on any iPhone out there, for those rare moments you actually use your iPhone as a phone phone.
Obviously, a noisy environment isn't the ideal testing ground, but for a snap impression, HD Voice is definitely better. We look forward to spending more time with it in the future, though, so we can do a fuller evaluation. HD Voice is also (or will be) available on the Galaxy S III, the Galaxy S IV, and the HTC One to name a few.