AT&T announced plans to begin supporting High Definition (HD) Voice over VoLTE in select markets beginning May 23.
Specifically, the VoLTE service will be activated on Samsung Galaxy S4 mini devices in select areas in Illinois, Indiana, Minnesota and Wisconsin. AT&T plans to add support for more devices and markets over time. Pricing was not disclosed.
http://about.att.com/story/att_introduces_high_definition_voice_in_initial_markets.html
