AT&T starts testing cross-carrier VoLTE calls

Looking back at 2015, AT&T mentioned some of the calling options it is now offering to subscribers. For example, iPhone users with iOS 9 can take advantage of Wi-Fi calling in areas where an AT&T signal is hard to come by. AT&T says that it plans to offer the feature to other operating services (read Android) in the future. The mobile operator started offering the service in October.
"I’m excited to share a big network milestone with you today. Our VoLTE network (Voice over LTE) covers more than 295 million Americans. That includes more than 27 million active subscribers – more than any other U.S. carrier. We’ve set up the network so customers in covered areas who have capable devices get instant access to VoLTE as soon as the coverage is live...you’ve been talking and surfing the web simultaneously on AT&T for years. But now VoLTE lets that happen at 4G LTE data speeds. VoLTE also lets us offer HD Voice. HD Voice produces more natural sounding audio by extending the frequency range of the audio signals, resulting in remarkably clearer calls."-Bill Smith, AT&T
source: AT&T via Engadget