Apple did not include the radio chip for voice+data on Verizon's LTE network.
That is incorrect. There is no "radio chip for voice+data" for LTE. LTE is a packet-switched data-only network, just like the internet.
Every LTE capable phone on the market today uses Circuit Switched Fallback Mode for calling, which switches off LTE and falls back to the carrier's legacy 2G/3G network. With an iPhone 5 on Verizon or Sprint, this means no simultaneous voice and data, as their primary networks (CDMA/EvDO) does not support it. Other phones on Verizon or Sprint get around this by having two baseband chips, one for CDMA and another for LTE, which is horribly inefficient (Google complaints about the HTC Thunderbolt for examples). The AT&T iPhone 5 on AT&T just happens to use GSM when it falls back, which obviously supports simultaneous voice and data.
The only true way to make a call on an LTE network is with Voice over LTE, which is VOIP (and early tests show it drains battery life just like running a VOIP app). No major carriers have rolled out VoLTE yet (Verizon says late 2013-2014 timeframe), and only a few phones on the market currently support it.