Verizon's long-awaited nationwide launch of voice-over-LTE (VoLTE) is slated to begin "in the coming weeks," bringing high-definition voice and video calling to compatible smartphones.

Verizon Wireless announced what it's calling "Advanced Calling 1.0" on Tuesday via a blog post. It promised a simplified contact list with video camera icons next to those who are equipped to receive video calls, which can easily be switched to from voice calls and vice versa. The audio for voice and video calls will also be delivered in HD, it says. (See Verizon VoLTE Testing Spotted.)

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To take advantage of VoLTE, both parties on the call must be Verizon customers on capable handsets. The carrier says it has been seeding the market with handsets, which it will VoLTE-enable with a software upgrade going forward. (See Apple Eyes VoLTE as 4G Voice Gets Real.)

Catch up on all the 4G voice advances on our dedicated VoLTE/rich communications channel here on Light Reading.

Verizon is the third of the big four to launch VoLTE. The technology has been in the works at the carrier for years, but has suffered many delays and setbacks. Unlike its competitors, Verizon is launching nationwide rather than in select cities. Because Verizon would eventually like to offer VoLTE-only calling, making sure its LTE network is up to the task is critical. For now, VoLTE will just be an "opt-in" choice on handsets, and Verizon customers will still have 3G calling as a fallback. (See Verizon Beefing Up Network for VoLTE, Multicast Video and Verizon Promises Voice-Over-LTE in 2014.)

AT&T Inc. (NYSE: T) has VoLTE up and running in parts of four states, while T-Mobile US Inc. claims nationwide coverage. HD voice champion Sprint Corp. (NYSE: S) has yet to commit to a VoLTE timeline, although the latest rumors have it looking at mid-2015 for a launch. New CEO Marcelo Claure promised at a recent town hall to have an answer in the future to an internal question about why T-Mobile has VoLTE and Sprint doesn't. (See Sprint CEO: Price Cuts First, Best Network Next , T-Mobile Expands VoLTE, Gets Going with 700MHz 4G, T-Mobile Beats AT&T, Verizon to VoLTE and AT&T Confirms VoLTE Launch in 4 States.)

— Sarah Reedy, Senior Editor, Light Reading