AT&T's voice-over LTE (VoLTE) service now reaches 295 million Americans, 27 million of which are active subscribers, the carrier said on Tuesday.

AT&T Inc. (NYSE: T) began rolling out the IMS-based 4G voice service on a market-by-market basis in May 2014 with four intial markets. It now covers more than 30 US states, save many of the mountain states like Wyoming, Idaho and Montana, as indicated in its coverage map. (See AT&T Confirms VoLTE Launch in 4 States and AT&T Promises More VoLTE Markets in 2015.)

AT&T only has around 14 million more of its 305 million LTE POPs still to reach with VoLTE, although it does have work to do on signing up more for the service. Out of its 87.6 million retail subscribers, only 27 million, or 31%, are actively using the service. AT&T has VoLTE set up so that customers in covered areas with capable devices are automatically switched over to VoLTE -- along with high-definition voice -- when their market goes live. (See VoLTE Triumphs (if Conditions Are Just Right).)

For more on HD voice and voice-over LTE, visit the dedicated VoLTE/rich communications section here on Light Reading.

Even for those AT&T customers in a covered area with a compatible device, VoLTE currently only kicks in when they are calling another AT&T customer who also meets both criteria. AT&T said at the end of last year that it would work with Verizon Wireless and other carriers on VoLTE interoperability between the networks, and Bill Smith, president of AT&T Network Operations, writes in a blog post today that, this month, AT&T saw its first VoLTE exchange between its customers and another carrier in limited, select areas. (See AT&T's Rinne: Carriers Working on VoLTE Interoperability and Verizon, AT&T Plan VoLTE Harmony in 2015.)

AT&T, Smith says, is working with other carriers on interoperability as well. T-Mobile US Inc. , like Verizon, offers nationwide VoLTE coverage and says that almost 40% of calls made on the T-Mobile network are now on VoLTE. Sprint Corp. (NYSE: S) has yet to launch any VoLTE markets, focusing instead on HD voice and WiFi calling. (See Sprint Extends WiFi Calling to the iPhone and T-Mobile Beats AT&T, Verizon to VoLTE.)

— Sarah Thomas, , Editorial Operations Director, Light Reading