That's the opposite of what I'm seeing though. AT&T's website shows substantial usage, and they've charged me $20 because of it.
I analyzed the usage from my trip home. It would appear Spotify is the primary culprit. It's offline mode still usages a fair amount of bandwidth. For what? I'm not sure. I think it might be streaming anyway, even though it should only be pulling from the local cache. I will have to look closer at the packets and distribution.
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To people seeing increased usage, what cloud based services do you use?
Edit: In the graphic above, 192.168.1.201 is my phone as assigned an IP by the VPN. This is of course not a very large sample (8ish MB), but it's still unexpected. I will try and track some more usage tomorrow.