Today, MetroPCS is announcing what it calls the "world’s first launch of Voice over LTE (VoLTE) services," beating out Korean SK Telecom’s claim  by just one day. For the most part, US carriers aren’t planning to adopt VoLTE on a wide scale until 2013, but MetroPCS announced its plan to move early during an earnings call in February . The company says its first VoLTE-compatible device was sold at one of its Dallas / Fort Worth stores.
Voice calls on older technologies like GSM and CDMA use old-fashioned circuit switching for voice calls, while VoLTE uses packet switching — turning voice into just another data type being transmitted over the carrier’s IP network. Hopefully the move toward more efficient transmission of voice data will let carriers bump up the audio quality .
source: the verge