United States-based carrier AT&T today officially announced its rollout of voice-over-LTE (VoLTE) calls. The VoLTE rollout comes in tandem with AT&T HD Voice launch, which provides better sounding phone calls. Starting on May 23rd, IllinoisIndianaMinnesota and Wisconsin will be the initially supported markets for these features, but AT&T says to expect a more complete rollout as time goes on:

Beginning May 23, AT&Ti is introducing High Definition (HD) Voiceii on an all-IP, Voice Over LTE (VoLTE) network in select markets. At AT&T, you won’t have to choose between faster data speeds and crystal clear conversations. HD Voice customers can simultaneously talk while surfing the Web at 4G LTE speeds, all on the nation’s most reliable 4G LTE networkiii – it’s the best of both worlds.

AT&T touts the features as being supported by Samsung’s Galaxy S4 Mini, but some of Apple’s products should also be able to support HD Voice. Back in 2012, Apple announced that the iPhone 5 would have HD Voice calling on supported carriers, so it is likely that iPhone users in the AT&T/HD-Voice-supported markets will be able to get better sounding calls. But what about voice-over-LTE support?

While the iPhone’s chipset can support calling over LTE, the iOS software does not actually have the ability to make calls over LTE networks. According to people briefed on Apple’s plans, this may change this fall with iOS 8 and the iPhone 6. As we reported a few weeks ago:

Another significant addition being considered for iOS 8 and the next-generation iPhone is voice-over-LTE support (VoLTE), according to carrier sources. Currently, when an LTE-capable iPhone needs to make a phone call, the actual call is placed over last generation networks such as 3G. With VoLTE, calls will be transmitted over the same type of network that LTE data is processed through, and this can allow for benefits such as improved call quality.

Of course, carrier support is needed for this functionality, and some countries around the world have carriers that have already rolled out support for VoLTE. For those in the United States, T-Mobile’s network (thanks to its agreement with Metro PCS) supports VoLTE while Verizon Wireless and AT&T are actively testing the functionality for a rollout later this year. Of course, it’s plausible that iOS 8 support for VoLTE will be pushed back if enough carriers are unable to meet the rollout timeframe.

Besides AT&T, major Japanese carrier (and close Apple partner) DoCoMo announced that it will begin supporting voice-over-LTE calling in June. As for other U.S. carriers, Verizon Wireless is actively testing voice-over-LTE functionality, and T-Mobile already supports it. T-Mobile also supports HD Voice functionality for the iPhone, as does Sprint, while the other major U.S. carriers are also actively testing support.