Simply put, it is Voice over LTE. It uses the same data stream for your calls as your internet browsing. Really hope Google releases an up...
The T-Mobile Samsung Galaxy S5 will be VOLTE-capable starting June 19 in the 16 cities that sport the voice-over-LTE upgrade.
Building on its data DNA, the Un-carrier also expanded Voice over LTE (VoLTE) to more than 100 million people in 15 total markets. T-Mobile was the first major provider to launch VoLTE, and now, because its network has been designed for data, is nearly doubling the amount of data dedicated to voice calls for highest fidelity, crystal-clear HD Voice and faster call setup times than ever before. The Un-carrier expects to deploy VoLTE nationwide by the end of the year.
Via: Android Central, Droid-Life