Are you aware of the data usage on your SmartPhone?

Smartphones can use up mobile internet without you even realising it. There are application which can still do a fair amount of downloading when they’re not open e.g. Gmail, Yahoo or Hotmail’s scheduled synchronization for new emails, Weather Updates, Application Updates, Social Networking applications’ updates etc. In short, you could easily end up using mobile internet & getting charged for it without even knowing it, regardless of whether you’re on Android or iPhone. Since it’s very expensive to use mobile internet over Base Rate when you are not subscribed to any mobile internet bundle, therefore it is really important you track how much data you’ve consumed while using different applications & web-browsing
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Internet Usage on Smartphones - Mobilink - Har Dil. Har Din.

The first page on all android settings shows detailed data usage and limiting by default on all phones.