Until now, the way voice has worked on handsets with a 4G LTE air interface has been, well, interesting. That is to say, thus far all voice calls haven't been over 4G LTE but conventional circuit-switched (as opposed to packet-switched) 2G or 3G networks like CDMA2000 1x in 3GPP2 land, or WCDMA/GSM in 3GPP land. For example, on Verizon LTE the handset has a second radio always camping 1x for voice and SMS, and on AT&T LTE the handset falls back from LTE to WCDMA for the voice transaction. This is CSFB - Circuit-Switched FallBack. LTE deployments thus far have focused primarily on data connectivity for a variety of reasons, and further logistical hurdles like emergency call support has made VoLTE effectively only possible...

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