Verizon appears to have confirmed that the iPhone 5 will not support simultaneous voice and data use on their networks, though it will work on AT&T. Both Sprint and Verizon tell The Verge
that simultaneous voice and data use won't work for the iPhone 5 on their networks, despite the fact that this hasn't been a problem with other LTE handsets.
Verizon's older EVDO-based handsets also couldn't do simultaneous voice/data, something AT&T used to have fun with in their marketing. Those ads may return; iPhone 5 users on AT&T's network will have no such limitations.
Verizon tells the New York Times
the design limitation was an Apple design decision:
Brenda Raney, a Verizon Wireless spokeswoman, said it was Apple’s decision to design the iPhone 5 so that customers could make voice calls and do Internet activity simultaneously only over Wi-Fi, not over Verizon’s cell network. “The iPhone 5 is designed to allow customers to make voice calls on the Verizon Wireless network and surf the Web on Wi-Fi,” she said in an e-mail. “It was an Apple decision."
Though the story has popped up in several locations, Brian Klug and Andandtech
is the only technology writer I've seen bother to explain why
the iPhone 5 definitely doesn't support either SVDO or SVLTE. It's as simple as looking at the FCC documents and the appropriate sections in the allowed and tested simultaneous transmitters section for SAR (Specific Absorption Rate) testing. There, it's spelled out that only one air interface can be active at a time, and that only one antenna can be selected for transmit at a time. There's also an explicitly called out mention to VoLTE not being supported. I didn't replicate the entire table of simultaneous transmission combinations which need to be tested (it is a huge table) there are no entries with CDMA Voice being active at the same time as any data mode save WiFi.
In short, with voice-over-LTE still not out of the oven, current Verizon LTE phones make calls over the older CDMA network. Apple didn't want to make another variant with the extra internals and antennas (adding weight, size) needed allowing this to happen -- so they didn't. Most carriers expect to have voice over LTE service seriously deployed by the second half of 2013.