Not to be outdone by AT&T, T-Mobile launches voice-over-LTE in Seattle on 3 smartphones

After years of languishing, voice-over-LTE (VoLTE) has suddenly become a hot topic. T-Mobile US(s tmus) has launched its next generation all-IP communications service in Seattle where anyone with an LG G Flex, Samsung Galaxy Note 3 or Galaxy Light can access it.

AT&T(s t) plans to take its VoLTE service live in Chicago and Minneapolis this week, though customers who want to access it must buy a new VoLTE-enhanced version of Galaxy S4 mini. T-Mobile customers with any of the three aforementioned Android(s goog) phones can now go into their settings and download the latest software update to make their devices VoLTE compatible. For now they’ll only be make or receive VoLTE calls in the Seattle area, but T-Mobile plans to bring the service to other cities throughout 2014.

T-Mobile is unique among the U.S. carriers because it already had a VoLTE service, gained when it acquired MetroPCS last year, but it has been phasing it out as it shuts down Metro’s old LTE networks. But in a T-Mobile blog post, CTO Neville Ray said that its new VoIP-powered communications platform would be better than anything currently out today because it is the first to support a new LTE-Advanced technique with a rather ungainly name: enhanced single-radio voice call continuity.

Mobilize 2012: Chetan Sharma - President, Chetan Sharma Consulting and Analyst, GigaOM Pro, Neville Ray - CTO, T-Mobile USA

T-Mobile US CTO Neville Ray (left) speaking with mobile industry analyst Chetan Sharma at Mobilize 2012 (c) 2012 Pinar Ozger [email protected]

eSRVCC, in short, allows the old 2G circuit voice service to work in tandem with the VoIP calls running over T-Mobile’s LTE network. When a customer leaves LTE coverage, the call doesn’t just drop. The connection is handed over to a 2G or 3G tower, and in the network core the IP call is punted over to the circuit-switched phone system. The idea is that customers notice nothing as their conversations move between old and new networks.

We’ll see AT&T and other GSM carriers adopt eSRVCC as well, but we won’t see it from CDMA providers such as Verizon(s vz) and Sprint(s s) since their 2G networks are incompatible with the technology. Verizon and Sprint will launch VoLTE, but their VoIP calls will drop as soon as they leave LTE coverage. That’s probably why neither is in a particular hurry to bring their VoLTE networks online.

Verizon has promised to launch VoLTE sometime this year (though it’s already pushed back the timing a few times), and while AT&T and T-Mobile have a head start, Verizon has said it won’t be piddling about with a pilot launches in a few cities. Instead it will launch its network nationwide and offer enhanced communications features such as video calling on top of VoLTE on day 1.