AT&T and Verizon are working together to enable interoperability between their voice-over-LTE (VoLTE) services, which they expect to achieve in 2015.
The carriers are both counting on VoLTE to be the primary, and eventually only, form of voice communications they offer on their networks. VoLTE works by using the Session Initiation Protocol (SIP) to initiate 4G calls, which are transmitted in data packets, with the IP Multimedia Subsystem (IMS) in the core network controlling the call session. Carriers like it because it allows for enhanced calling features and high-definition voice and, most importantly, will eventually let them refarm their 3G spectrum when they no longer need it for voice fallback.
The problem, however, is that all the carriers run LTE on unique spectrum, so their flavors of VoLTE don't work together. A Verizon Wireless customer calling an AT&T Inc. (NYSE: T) customer over VoLTE drops back to 3G voice, and vice versa.
That is what AT&T and Verizon say they are working to change, reiterating comments AT&T's Kris Rinne made to Light Reading in September. The carriers said on Monday that their engineers are "working through a full set of requirements, beginning with extensive testing in lab environments and then moving to field trials." The pair is working closely with the GSM Association (GSMA) to achieve this, and say they will begin to work on adding other rich communications features once they've nailed interoperability. (See AT&T's Rinne: Carriers Working on VoLTE Interoperability.)
Verizon is calling AT&T its first interoperating carrier for VoLTE, but both plan to work with others going forward. T-Mobile US Inc. actually has the biggest VoLTE footprint to date, but it was not mentioned in today's announcement. Sprint Corp. (NYSE: S) has plans to launch VoLTE, too, but hasn't yet committed to a timeline. (See Apple Eyes VoLTE as 4G Voice Gets Real, Will Apple Enable VoLTE for the iPad? and T-Mobile Beats AT&T, Verizon to VoLTE.)
AT&T has had VoLTE live in parts of four states since May, while Verizon flipped the switch on VoLTE across its footprint in September. T-Mobile claims to have completed its nationwide 4G voice footprint in July of this year. (See AT&T Confirms VoLTE Launch in 4 States, T-Mobile Expands VoLTE, Gets Going with 700MHz 4G and Verizon to Launch HD VoLTE in 'Coming Weeks'.)
— Sarah Reedy, Senior Editor, Light Reading