Streaming video is going to be your number 1 enemy. Most images are optimized for the web, so that should help some, but I’m not sure about hour-long Google Voice calls (they may have quality settings that can help there). I know iPhones can frequently get around 2G’s of data in a month, but that’s ignoring all the data used via Wi-Fi connections, the fact that you’re viewing smaller images and websites that typically have fewer ads. Also you should probably factor in regular software updates, which can be very significant in size. I’m so used to the all-you-can-eat mentality with data that it’s really hard to fathom just how awful that must be.