T-Mobile users have enjoyed the flawed, but promising feature for some time now

Sprint Corp. (S) -- the third largest cellular carrier in the U.S. -- has added support for Wi-Fi calling and HD Voice (wideband audio) for the HTC One M8, the flagship smartphone of Taiwan's HTC Corp. (TPE:2498).  The One M8 is one of the top high-end Android phones at Sprint, who (as "Gor-don" would tell you) carries a special "Harman/Kardon Edition" of the device.
Rival budget carrier T-Mobile U.S. Inc. (TMUS) -- the nation's fourth largest carrier -- supported Wi-Fi calling and texting from launch and HD voice from launch.  Verizon Wireless (the nation's largest carrier), AT&T, Inc. (the nation's second largest carrier), and Sprint all blocked Android v4.4.3's built-in support for Wi-Fi calling at launch.
The addition of Wi-Fi calling on the Sprint network will be welcome news for some users, especially given Sprint's relatively poor performance (compared to competitors, including T-Mobile) inside tall buildings in urban areas.

HTC One M8 HK Edition
The Sprint HTC One M8 harman/kardon edition

The upside to Wi-Fi-based telephony in Android KitKat is that you can call and send texts in areas with little or no service, but with ample Wi-Fi access.
The downside is that when you leave the area of Wi-Fi coverage the call drops, as there's no support for a handoff procedure between cellular and Wi-Fi calls.  By default, most users' devices will automatically connect to known Wi-Fi networks (when available) and use them.  Hence, some users will find their calls dropping when they make a call just before they home (and their home's Wi-Fi network).  As there's no fine grain control over whether to place a specific call over Wi-Fi or the cellular network, the only real quasi-fix at this point is to disconnect from your Wi-Fi network prior to placing the call, if you expect to be leaving the network area before you finish the call.
Beta releases of iOS 8 have added Wi-Fi calling with similar upsides and downsides.  AT&T and Verizon have hinted that they may soon allow the feature in Android Kit Kat devices.
HD Voice is a separate technology, which is implemented by a growing number of OEMs in premium devices.  Typically, smartphones used a narrowband filter, only capturing human speech in the 300 Hz to 3.4 kHz.  As the natural speech range is 80 Hz to 14 kHz in an average human, this cuts off some of the richness of the human voice and makes people sound -- for lack of a better phrase -- not quite like themselves.  Wideband audio, leverages the fact that technologies like HSPA+ and 4G LTE can stream large amounts of data on-demand to expand the real-time calling range to 50 Hz to 7 kHz.  
The technology is advertised under the trade name "HD Voice".
The update from Sprint still sticks with Android v4.4.3.  The firmware version is 2.16.651.4.  The Google Play Edition of the HTC One M8 is the only version of the phone to currently have Android v4.4.4.

HTC roadmap
The carrier HTC One M8 will get Android 4.4.4 in July-August. [Image Source: @LlabTooFeR via PhoneArena]

As seen above, a roadmap leaked at the end of June indicates that the HTC One M8 on the four top U.S. carriers will receive the update to the latest version of Android sometime later this month, or by the end of August at the latest.  Of course, it's then up to the carrier to distribute that update in a timely fashion.

Source: Pocketables

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