I'm the proud owner of a (sim lock free: so no provider issued software) Lumia 920, and got a bit of a shock receiving my latest provider bill. I'm supposed to have used about 20 MB data using cellular data connection (from here on CDC)! Since I don't have an internet deal with my cellular provider, this has been a bit of an expensive surprise.
I've set my lumia to NOT use CDC and set wi-fi ON, and I know for sure not to have have used internet out of range of a wi-fi connection. So where does the 20 MB come from?
I know now, but I'm not sure why:
e-mail sync uses CDC, not wi-fi, even though windowsphone.com says it'll prefer wifi over CDC. Then why is it ignoring my settings?
Browsing through all setting options I decided to turn my cell to 2G. Know what? I couldn't sync email (hotmail, exchange, etc) at all anymore! The error I received said that I didn't have CDC (I told my phone that I knew that because I turned CDC off... no response )
Solution to that was to disable wi-fi, go back to start and go back and enable wi-fi.
But the question remains: why does the lumia 920 use or prefer cdc over wi-fi for syncing emeails, even though I've turned cdc off???
It does not prefer Cellular data over Wi-Fi, but will disconnect from Wi-Fi when under lockscreen and not connected to a charger. This is well documented behaviour.
The Portico update currently rolling out for WP8 devices will have an option to keep Wi-Fi on when under lockscreen which should resolve your issue..
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