MetroPCS said late Tuesday it had launched a commercial voice-over-LTE service, claiming a win in the international race for the next generation of mobile voice.

The fifth-place US carrier said it was making VoLTE-capable phones available for the first time and had made the first sale of a VoLTE handset at a store in the Dallas-Fort Worth, Texas, area. The company, which has about 9.3 million subscribers and a network that reaches 280 million people, said it would continue to roll out VoLTE services and handsets in the coming weeks.

VoLTE sends voice calls over LTE, an IP (Internet Protocol) packet data network, instead of the circuit-switched connections that are typically used on cellular networks including 3G. By converting voice to data, carriers can make better use of their radio spectrum by using the higher efficiency of LTE and ultimately reusing 3G spectrum for the new network. It can also allow for better voice quality.

MetroPCS said the technology will also form the basis of future RCS (Rich Communication Services) offerings later this year. RCS can include a variety of real-time services such as voice and videoconferencing.

MetroPCS' announcement looked to be an attempt to keep two South Korean carriers, SK Telecom and LG Uplus, from claiming the first VoLTE launches. On Tuesday, those carriers said they would start VoLTE services on Wednesday. MetroPCS sent out its press release just before 5:30pm Eastern time, which was around 6:30am Wednesday in South Korea. In South Korea, the Samsung Galaxy S III and LG Optimus II are the only VoLTE devices on the market.

The VoLTE phone MetroPCS sold on Tuesday was an LG Connect 4G Android smartphone, which it said is available at select US stores. The carrier said it would continue to roll out VoLTE services and phones in the coming weeks.

Though VoLTE uses a different underlying system for making and carrying calls, it is designed to be transparent to subscribers. MetroPCS said the LG Connect 4G Android could use VoLTE for calls to and from the phone, regardless of what handset or network was on the other side of the connection.

MetroPCS calls itself the fifth-largest US operator with its own network, measured by the number of subscribers it serves. The next largest carrier is T-Mobile USA. MetroPCS also launched the first commercial LTE network in the US, in Las Vegas in September 2010. Today, its LTE network is available over about 90 percent of the coverage area of its 3G CDMA network.

Of the largest US operators, both AT&T and Verizon Wireless plan to launch VoLTE services, but they aren't expected until 2013. Sprint is just beginning to roll out LTE, and T-Mobile plans to offer it next year.

Meanwhile, the UK is lagging far behind. Wireless company UK Broadband switched on the first wholesale 4G network in the London Borough of Southwark in March, but no commercial services are planned until after Ofcom's spectrum auction at the end of the year.