I have just received my new phone ( a Nokia Lumia 520). When i check my data usage with the self serve option, it shows that my phone is using data even when my phone is turned off. What could be causing this to happen?
You may have data enabled on the 520, to turn off data go to Settings > Cellular - tap Data connection and select 'Off'. I'm assuming you're using Wi-Fi to check your usage, if not, you're using data to check data usage. Any apps that update in the background like weather, Facebook, email sync, run in the background and also use data. It's best to turn off data and use Wi-Fi whenever possible.