So basically 42% of 66% of all phones in US...meaning about 28% of all phones in US.T-Mobile was rolling it out May of 2014:
http://support.t-mobile.com/communi...obile-begins-rolling-out-voice-over-lte-volte
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http://appleinsider.com/articles/14...sents-42-of-smartphones-owned-in-the-us---npd