The HD Voice technology promises to make phone conversations crystal clear, and the HTC EVO 4G LTE was one of the very first phones to support it, at least in America. But you need a carrier to support it as well, and although Sprint promised tp make it available later this year, the phone itself launched without that functionality working.
Sprint is saying again that the feature will be supported by the end of the year, but there's just one other problem. If you want to experience phone conversations in HD voice, both the caller and the receiver need to have phones that support this feature. Since the HTC EVO 4G LTE is the only one supporting HD Voice right now, that means that everyone has to have this phone.
To make this work – and fast, before people stop being excited about it – Sprint needs to launch a whole range of HD Voice-enabled phones. Then there's another problem. While Sprint is trying to make the old network work with HD Voice, the other carriers are looking into VoLTE, which is Voice over LTE, a technology that should also make voice calls very clear, much like Google Talk or Skype calls, since they are sent over the Internet.
So where does this leave Sprint? Unless the HD Voice add-on is very cheap for smartphone manufacturers, Sprint will probably be left once again on its own with an alternative technology that nobody else is going to use in that given market, much like when it chose WiMax over LTE. On that note, Sprint is finally deploying LTE, but there's no word about the carrier adopting VoLTE yet.