Your phone will regularly contact the network to update and synchronise it's data settings. This could happen, e.g. whenever you turn your phone on. This happens before your phone has looked at your settings to see if you have mobile data turned on or off.
Both Apple and Android phones have the option to turn off mobile data usage. However, this only turns data off for App use. The phone and operating systems (iOS or Android) still have some data requirements that cannot be disabled in the user settings. These are features like 'Find-my-phone' and 'App/Play store' verification.
Some simple/feature phones also have some data services, (and some only in their operating system and firmware), that are used to check for updates and network registration.
If you do not wish this to occur, and have mobile data turned off on your phone, on some phones (particularly newer iOS versions on the iPhone) it is worth turning off 4G as well.
This is a very small amount of data, but if you haven't used any more data that day it still charged 1p for the daily session.
If you would like to turn mobile data off completely you can do this in your account area. Please log into your account and go to network settings.
Please make sure that your phone is fully authorised with iTunes/iCloud. If the activation and authentication to these services and does not compete successfully then your phone will continue to try and complete the authentication even with mobile data turned off in the handset.
The Sony suite requires the phone to be authenticated and will attempt to do so every time a new SIM is inserted in the phone. Please ensure that this authentication has completed as the phone will keep trying until successful.