Bad news for Sprint users. It has now been confirmed that Sprint GFNII does not support SVDO (simultaneous voice and data) over 3G. You need to be in an LTE area for voice over 3G and data over LTE. The GFNII fundamentally uses a similar design concept to the iPhone 5 in terms of antennas. As with the iP5, an additional antenna could not be fitted in the device just for the Sprint and Verizon versions of the devices, and so SVDO over 3G/CDMA is impossible.
As a side note, the reason the US GSIII have SVDO over 3G on Sprint and Verizon is because the US GSIII uses Qualcomm Snapdragon S4 with the integrated LTE modem instead of Exynos + additional LTE modem. By using Exynos in all versions of GFNII, this issue is created for that phone which doesn't exist in the US GSIII. I'm not smart enough to properly understand and explain why exactly this is the case,
but this blog post about the Sprint GFNII explains it well.
For Verizon users, who have LTE in over 450 markets covering 75% of the US population, this isn't that big a deal. You will get
voice over CDMA and data over LTE. One advantage of having Verizon 4G LTE on your Verizon GFNII, I suppose. It even makes the astonishing Verizon button on our Verizon GFNIIs almost bearable.
For AT&T users outside of an LTE area, you still get SVDO over 3G+HSPA, the same as it has always been for people on GSM networks. For T-Mobile users, this whole post is entirely irrelevant to you since T-Mobile has not begun deployment of LTE.
Verizon