Simultaneous Voice and Data - how and why
There are complaints floating around the web about how the iPhone STILL doesn't support simultaneous voice and data on Verizon/Sprint. Some think it's the carriers' fault. Others think it's Apple. Here's an explanation of what's going on and what will cause this to change (Hint: iPhone 5, 5s, and 5c should all get simultaneous voice/data "soon").
In the beginning, it was GSM versus CDMA. Neither of these technologies in their 2G incarnations (Edge, CDMAone) support simultaneous voice and data. They can transmit voice over that link, but they need a second link (wifi) for data.
For 3G, there is no true GSM standard. CDMA branched off to CDMA2000 (what Verizon/Sprint use), and WCDMA (UMTS, most of the world's carriers, which many now consider the evolution of GSM due to its widespread adoption by GSM carriers). CDMA2000 supports more simultaneous connections within a frequency range and better distance to a connection, allowing for more consistency, more local users, and better coverage. This is a large part of why Verizon's network was so damn good in the heyday of 3G. UMTS supports better DL/UL speeds. It also natively supports "SVDO" on a single link. This means if you have a 3G/H/H+/4G icon on your GSM-based phone (AT&T, T-mobile, etc.), then you have support for simultaneous voice and data. CDMA2000 still requires that second link.
For CDMA2000, there is an SVDO option that doesn't require wifi. The phone has to support two links at once, requireing a dual antenna design. The HTC Thunderbolt was the first to use this, and battery life suffered. Most of Verizon's phones have since avoided this approach because of the horrid battery life. (Yes, the Thunderbolt could do simultaneous voice/data without an LTE connection)
With the advent of LTE, many felt that this would solve CDMA carriers' simultaneous voice/data problems. It sort of has. On a CDMA carrier, like Verizon, an OEM can make their phone conduct voice over CDMA2000 (3G) while data goes over LTE. This will HEAVILY drain your battery, unfortunately. On GSM-based carriers, this mode is not supported. Even if you have a strong LTE signal, while on a voice call, your phone will fall back to UMTS (3G/H/H+/4G) and conduct simultaneous voice/data over that single link. This provides significant battery savings. Apple decided to use this approach for their iPhone across the board, regardless of carrier. So, if a carrier cannot support a single-link SVDO option, it won't work on the iPhone.
But there is hope. Once VoLTE (Voice over LTE) becomes functional, we'll have simultaneous voice/data over a single-link LTE connection. This should work with a software update on most current LTE devices, provided the OEM offers such a software update (Apple and Samsung? Yes. Everyone else? Likely no for phones over a year old). Verizon expects to roll out VoLTE trials by December. This will also offer battery savings as the CDMA/UMTS radios can be in standby in areas of good coverage, something that will benefit Verizon and even AT&T over carriers like Sprint and T-Mobile (where their LTE deployments are more patchwork).