You can already get high-quality LTE phone calls on multiple US carriers, but they're not all that practical when they only work between customers from the same carrier. They're on the cusp of hitting the mainstream, though: AT&T has quietly revealed that, as promised, it recently completed the first Voice over LTE calls between its customers and another network in "select areas." It's a bit late to the game (T-Mobile started cross-carrier testing a while back), but this will be a big deal when it eventually rolls out nationwide -- you'd be much more likely to get LTE quality when you call a friend.
As it stands, the service is no longer a rarity within AT&T's own network. The provider says its VoLTE is now within reach of 295 million Americans, or about 96 percent of its total LTE coverage area. That's wide enough that you can simply assume that improved calling will be available anywhere you get an LTE signal. In other words, inter-carrier LTE calling is really just the last piece in a mostly completed puzzle.
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