T-Mobile launches VoLTE (not nationwide), only the Galaxy Note 3, Galaxy Light and LG G Flex support it for now
With VoLTE, T-Mobile customers will be able to use LTE data during phone calls. HD Voice is also one of the features that's usually enabled by VoLTE (although this was offered by T-Mobile even without VoLTE). Another benefit of the new technology is that it allows for faster call setup times.
AT&T should launch VoLTE services tomorrow in select markets, while Verizon intends to do it sometime later this year. MetroPCS (which is now owned by T-Mobile) was the first in the US to offer VoLTE services, starting 2012.