It's been many years in the works, but Verizon is today announcing its first official rollout plans for its voice-over LTE, or VoLTE, service. VoLTE, which has already been launched in limited markets by AT&T and nationwide by T-Mobile, utlizes LTE data networks to carry voice traffic, unlike traditional cellular networks that have different streams for voice and data services. Verizon's launch of VoLTE, set to begin in the nebulous timeframe of "the coming weeks," will be nationwide, unlike the limited rollouts by AT&T. But the carrier is restricting it to select devices and requiring customers to opt-in to use VoLTE before it making it more widely available across its 100 million-plus customers.

Verizon's VoLTE service will work wherever the carrier has LTE data coverage, which is most of the United States. In order to support the new service, Verizon says its network had to be substantially upgraded, and it wasn't going to roll out VoLTE until the experience was up to its standards and customers' expectations, which is why the launch was delayed numerous times.

In addition, Verizon's launch of VoLTE brings the carrier's first HD voice service, something that T-Mobile and Sprint have offered in various capacities for years. Verizon's HD voice uses a 13kbps codec (standard CDMA voice is about 8kbps) verses the 24kbps codec T-Mobile uses, but Verizon feels that it wasn't necessary to use the higher bandwidth to acheive the greatly improved voice quality it was looking for. Though Verizon is using an industry standard for HD voice, it will initially only be available between customers that have compatible smartphones on Verizon's own network.

Users will need a compatible handset, a software update, and to opt-in on their accounts in order to use VoLTE

VoLTE also brings an integrated video calling service, which Verizon customers will be able launch right from their phones' dialers. Video calls can be initiated during a VoLTE voice call, and callers will be able to switch between the two at will. Since Verizon's video calling service is integrated into a phone's dialer, it doesn't require users to sign up for a separate service, such as Skype or Google Hangouts.

Example screens of a VoLTE video call on a Verizon smartphone

Though Verizon says it will be rolling out VoLTE service in the near future, it's not yet saying which of its handsets will be compatible with the service. Customers that are interested in using VoLTE will need a compatible smartphone that has received a software update to enable VoLTE and will have to opt-in to the service on their account to make it active. Verizon's pitch to launch VoLTE as an opt-in service centers around customer choice, but it's hard to not see that the carrier is hedging its bets against a new service potentially causing issues for customers if they were forced to use it against their will.

I was able to test Verizon's VoLTE, HD voice, and video calling services (Verizon is referring to the whole package as "Advanced Calling 1.0") in New York City on an LG G2 that had been updated to work with VoLTE. Calls over VoLTE were significantly better sounding, without the audio compression and distant feeling that most cellular calls have. Switching between a VoLTE call and a standard CDMA call was like night and day, with the VoLTE call just sounding a whole lot better. I'm not convinced that it will make me actually start using my phone for voice calls again, but if I had to, I'd choose a VoLTE call over traditional cellular any day of the week.

Call quality over VoLTE was significantly better than standard cellular service

The integrated video calling service was straight forward and easy to use. Switching between voice and video calls was painless and free of hiccups, and the quality was on par for other video calling services that can be used over LTE networks (but not as good as FaceTime over Wi-Fi, for example). Verizon's vice president of network operations, Mike Haberman, says that deep integration of video calling could drive users back to making calls as opposed to using other forms of communication, such as email, texts, or other messaging services. Video calling provides a "deeper level of interaction with the other person," says Haberman.

Verizon's initial launch of VoLTE has been long awaited, but it's likely that most of the carrier's customers will still be waiting a while before they're able to take advantage of it. Verizon would not comment on whether or not the iPhone will support its VoLTE, HD voice, and video calling services, but VoLTE has been earmarked as a feature of iOS 8, which is expected to be officially launched next month. Still, customers will need the right phones, the right software updates, the right features on their accounts, and be calling the right other customers in order to see VoLTE's benefits.

The promise of full voice-over-IP cellular calling isn't quite here with Verizon's first VoLTE rollout (the carrier has no plans to shut down its CDMA network any time soon), but it does take us one step closer to getting there.

Correction, 8:45AM ET: An earlier version of this article stated that T-Mobile's VoLTE service was only available in limited markets. It is now available nationwide and the article has been adjusted to reflect that.