I believe Verizon's implementation of cdma does not allow data to work while on a voice call. Wifi will work, however, and you can be on a call and browse over wifi. At some point, you may be able to use 4g while on a call, but I don't think that has been turned on yet by Verizon.
I'm in a 4G area(san diego). I started having this issue after updating to the new software. I used to tether and be on a voice call at the same time!....