MetroPCS has become the first operator in the country to launch voice service on an LTE network.

The operator began selling a voice-over-LTE (VoLTE) model of its LG Connect 4G smartphone in some of its retail stores on Tuesday.

The Android device has been available for some time, but up to this point was not loaded with a VoLTE client. Going forward, all new LG Connect units shipped to customers will be VoLTE-capable, a MetroPCS spokesman said.

"We are working to fill channels in all markets and online with VoLTE-capable LG Connects, which should be happening shortly," the company spokesman said.

Beyond the Dallas/Fort Worth market named in its announcement yesterday, the spokesman declined to specify which cities were carrying the VoLTE phone. The LG Connect currently sells for about $250 on the MetroPCS website.

MetroPCS' LTE network spans 14 markets, covering about 90 percent of its CDMA footprint. The prepaid provider plans to introduce additional VoLTE phones in the coming weeks.

The release of the smartphone fulfills MetroPCS' recent pledge to introduce a VoLTE handset during the third quarter. It had first planned to debut its VoLTE service during the first quarter but the launch was delayed until the second half of this year.

VoLTE calls are placed the same way as calls over MetroPCS' legacy network, so subscribers using the new LG Connect may not notice a change in their voice service. Customers with VoLTE phones located outside MetroPCS' LTE footprint will have their voice calls routed over CDMA.

Moving to VoLTE is a matter of urgency for MetroPCS, which faces significant spectrum constraints. It needs to shift customers to its more efficient LTE network so that it can free up spectrum currently dedicated to supporting voice and data services on its CDMA network, repurposing it for LTE.

"The initial move to LTE and this move to VoLTE allow us to achieve significant spectral efficiencies and will increase network capacity so that we can enhance the 4G LTE experience that our customers have today at a tremendous value," MetroPCS CEO Roger Linquist said in a statement.

Attempts to get customers off its legacy network have been held up by a lack of support for VoLTE and the limited availability of affordable LTE smartphones. LTE smartphones priced between $100 and $150 are expected to be introduced by MetroPCS this month.

Just 8 percent of MetroPCS customers, about 700,000 subscribers, used LTE smartphones at the end of the second quarter.

Linquist said the VoLTE service provided a foundation for MetroPCS' upcoming Rich Communication Services (RCS), scheduled for launch later this year. RCS is seen by some as a means for operators to compete with services from over-the-top providers.

MetroPCS, which lost 186,000 net subscribers last quarter, is working to compete against larger national providers with stronger networks and low-cost prepaid providers like Cricket Communications.

The launch of VoLTE and RCS will help MetroPCS "differentiate itself from other MVNO's and other service providers" by giving its prepaid customers access to postpaid-quality services, Linquist said during the company's recent earnings call.