Brian X. Chen, writing for The New York Times, explains why:
Older Verizon and Sprint smartphones on 3G networks were not able to handle simultaneous calls and data because of a limitation in CDMA, the 3G technology that those networks use. But now some Verizon 4G LTE smartphones will let you stay on a phone call while looking up something in an app or checking e-mail. So why not the iPhone 5?
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.”
The explanation for this, it turns out, is complicated. The technology in 4G LTE networks does not currently handle voice transmissions; it only does data. So when you place a phone call on a 4G LTE smartphone, it’s actually rolling back to the carrier’s older second- or third-generation network, according to AnandTech, a Web publication that does deep analysis on hardware.
The ability to talk and surf simultaneously is one of the reasons I’ve stayed with AT&T. I know most people don't care about such a thing, but I do; I find it really convenient, and do so often. The other reason is, frankly, I’ve never had problems with AT&T in my 7 years with them. Where I live in the Bay Area, coverage is good and the data speeds fast.
Were I to ever switch carriers, it’d most likely be on principle, as AT&T has been notoriously slow and dick-headed on issues like not supporting MMS right away and restricting FaceTime over cellular. The latter move is especially fucked up considering Verizon’s supporting it without the need for a special plan. Again, it’s the principle of the matter -- leeching its customers for even more money -- that would persuade me to switch networks. Otherwise, AT&T’s been fine for me.