It's only an issue for non-GSM phones i.e. CDMA phones used in the US and parts of Asia. CDMA does not allow data and voice to run at once over the same radio, while GSM does. Since LTE is an entirely IP network, voice can be run over it like any other IP network. Current phones/carriers do not support it though, but there's some talk that it will be introduced in the next year or so.
Two of the major carriers in the US use CDMA (Verizon and Sprint), while the other two major carriers use GSM (T-Mobile and AT&T). I'm having some trouble finding recent market share numbers for them, but it's mostly half and half.
