Will an Android phone still use data when cellular data is switched off? I refer to the option under Settings / Data Usage. If that is turned off is there a possibility for the system to still use mobile data?
If that is turned off is there a possibility for the system to still use mobile data?


Absolutely not. That setting turns off the data completely as soon as you use it. 

Sometimes it takes 2-3 seconds to turn the data off completely(the LTE/H+ sign to disappear because of network issues), else the data is always switched off immediately.