Verizon Wireless has 4 million customers using voice-over-LTE (VoLTE) today, and it's working aggressively to move it from an opt-in feature to an out-of-box offering and, eventually, to the only voice option on LTE phones.

Speaking at an Oppenheimer conference on Tuesday, David Small, executive vice president of wireless operations at Verizon Wireless , said the carrier is going through a natural evolution when it comes to 4G voice, which it flipped the switch on nationwide in September of last year. (See Verizon to Launch HD VoLTE in 'Coming Weeks'.)

Currently, it's an opt-in feature that's being suggested by Verizon employees when customers buy a new device. Verizon is, however, working "aggressively" to make VoLTE the default calling option on new phones out of the box. The end goal, of course, is to be able to offer LTE-only phones and refarm its 3G CDMA spectrum for LTE. (See Verizon Begins 3G Refarming to Add LTE Capacity.)

That is still a ways off -- Small said it would happen when the carrier could guarantee a "Verizon Wireless-like experience" with voice. In the meantime, it has at least been able to get "much, much more attractive pricing" on those handsets that have CDMA in them. "We really get that benefit," he said.

VoLTE runs over Verizon's 4G data network, but Small said all the data consumed for voice is mediated out, so consumers won't see an additional charge for it.

For more on 4G voice, visit the dedicated VoLTE content section here on Light Reading.

To realize the benefit of improved voice quality and call set-up times on the 4G network, both users have to have VoLTE-capable Verizon handsets and have gone into their settings to turn the feature on. At present, it's not the most obvious process, but Small said that almost 4 million Verizon customers have flipped the switch. (See Verizon, AT&T Plan VoLTE Harmony in 2015.)

Verizon has a team of system performance engineers dedicated to testing VoLTE with handset providers, in addition to the 100 employees it has doing drive tests across the country. The service is performing better than its traditional 3G voice, he said, and isn't receiving any more call-in complaints than non-advanced calling. Customers value the experience, Small said, but added that Verizon's overall call quality has always been better than its competitors. (See VoLTE Triumphs (if Conditions Are Just Right) and Verizon VoLTE Testing Spotted.)

"From the time we launched our voice service until the time we [got a] drop call rate of .4, which is a good place for consumers to be, it took us quite some time to get there -- many, many years," Small said of 3G voice. "It took us half the time to get to pretty close performance metrics with VoLTE or advanced calling. We feel very good about the service."

— Sarah Thomas, , Editorial Operations Director, Light Reading