The VoLTE roll out in the United States has been staggered at best due to limited availability. Consumers with the ability to use VoLTE can place HD Voice calls, 1-way or 2-way video calls and use voice/data services at the same time. Unfortuately, VoLTE only works when both parties have supported devices from the same carrier.
When Verizon announced VoLTE availability, customers could only use such service if both the sender and receiver had two devices, either the Samsung Galaxy S5 or the LG G2. AT&T offers VoLTE only on the Samsung Galaxy S4 Mini and only in Illinois, Indiana, Minnesota and Wisconsin. T-Mobile offers VoLTE on the Samsung Galaxy S5, Galaxy Light and Note 3 and the LG G Flex.
Now, AT&T and Verizon have agreed that they will soon have VoLTE interoperability between their customers at some point in 2015. This means that customers of AT&T and Verizon will have the ability to send VoLTE HD Voice calls to customers of the other carrier. Although VoLTE HD Voice calls will be the first part of the interoperability, other features will also be added by both carriers in the future.
As Verizon pointed out in their press release, engineers from both companies are working on this project and must go through extensive testing in lab environments and then onto field trials.