T-Mobile US says it’s the nation’s leader in voice-over-LTE calls on its mobile network. This week at the Morgan Stanley Technology, Media & Telecom Conference, T-Mobile US CFO Braxton Carter said more than half the voice calls on the carrier’s network are on VoLTE and the “un-carrier” is ahead of its competitors in VoLTE penetration.

Carter said T-Mobile US is in the process of repurposing legacy spectrum for LTE. The company is also expected to bid aggressively this year in the 600 MHz spectrum auctions, but any spectrum T-Mobile US secures through that process will not be available until the TV stations currently using the spectrum move to a new channel or go dark.

T-Mobile US has been loud and proud about VoLTE since launching the service in May 2014. The company was the first U.S. carrier to launch VoLTE and has been adding markets and devices quickly. By the end of last year, T-Mobile US said almost 40% of its calls were on VoLTE, and now it says that number is above 50%.

Verizon Wireless and AT&T Mobility have also launched VoLTE within the past two years, while Sprint has said it was currently more focused on voice-over-Wi-Fi rather than VoLTE. So far, AT&T Mobility is the only carrier claiming support cross-carrier VoLTE calls.

VoLTE on T-Mobile US got a boost with the launch of the iPhone 6s and 6s Plus last fall. T-Mobile US told users last fall it was so confident about the performance of the new iPhones on its network that it would offer refunds to any customers who were not satisfied.

VoLTE enables high-definition voice calls, provided both parties have compatible handsets and are in LTE coverage, and also facilitates call handoff between LTE and Wi-Fi networks. This is particularly important to T-Mobile US, which relies heavily on Wi-Fi offload for both data sessions and voice calls.

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