Sony issued a blog post reminding everyone that the Xperia T is HD voice certified. Apparently, this feature was not listed on Sony's product page for the device originally, so people kind of forgot that it was a feature of the device. It has since been added to the Xperia T's page on Sony's site.
HD Voice is one of the biggest advancements in call quality since the invention of, well, the cell phone. When we talk about new phones and their features, we talk about processors, RAM, storage, and operating systems, but all too often, we gloss right over how well the phone makes calls. HD Voice is a feature designed to bring calling to forefront, and Sony reminded us of that with its phone.
HD Voice works best when both parties have phones with the feature and carriers that support it. In the UK Three and EE (Orange + T-Mobile) support HD voice, and it's supported on some other carriers with the European phone. People on these networks have had the feature since the phone hit the market recently, but they may not have even realized it was there.
Sony also posted a link to the a demonstration of HD Voice so you can hear for yourself how much better it sounds. In the demonstration you can really hear how well HD Voice blocks out the background noise. It's quite impressive. It's good to see Sony actually using call clarity as a major selling point for the device and remembering that a major purpose of a cell phone is being just that, a phone.