My iPhone was consuming large amounts of Cellular Data while being connected to a WiFi network. It was consuming several GB in a few days specially System Services > Documents and Sync.
Note: In case other app is consuming your data you will be able to see it in this section. A possible solution is to reinstall that application or disable it for using cellular data.
After several days trying to figure out how to fix it. I came to a solution without knowing exactly what caused this problem.
To know how much data is being consumed go to Cellular Data all the way to the bottom to the section Reset Statistics, then monitor it for a few days. If you detect that is consuming large amounts of System Services Cellular Data, you should do the following Reset your Network Settings.
Go to Settings > General, all the way to the bottom to Transfer and Reset iPhone, then click on Reset > Reset Network Settings. It is going to ask for your password, the iPhone will reset it self.
By doing this you will need to enter WiFi information for the networks that you wish to connect.
This is what solve my problem.