Don't know how well known this is but this just popped up on engadget
http://www.engadget.com/2015/04/22/three-voice-over-lte/?ncid=txtlnkusaolp00000602
http://www.engadget.com/2015/04/22/three-voice-over-lte/?ncid=txtlnkusaolp00000602
The provider is currently on track to switch its VoLTE service on in September, facilitated by tagging additional 800MHz spectrum onto its existing 1800MHz network. This new slice of bandwidth is expected to significantly improve coverage in problem areas, particularly inside buildings,