The phone isn't actually in anyone's hands yet, but most likely VOICE will continue to route through the 3G Microcell, and DATA will route through WIFI first, LTE second, or the 3G Microcell last, depending on availability.  To the best of my knowledge, the iPhone5 does not support Voice Over LTE (VoLTE), and in fact, neither does AT&T's network, so all voice calls will continue using UMTS/WCDMA ("3G")

Hope this helps!