My Android 10 phone seems to be using mobile data even when mobile data is turned off:
The setting called "switch to mobile data automatically" is off FYI. The data, in very small quantities, is used by different apps including Facebook, Skype, Gmail and Google assistant; when the Wi-Fi is disabled, connected-no-internet or out of range.
How can I make sure the mobile uses mobile data only when mobile data setting is on?
On modern devices like yours "mobile data" is not only used by apps.
Instead basic services like phone calls are also transmitted as "data". This is calls Voice over LTE (VoLTE). In a LTE network this is the only way to do voice calls, there is not voice network anymore. LTE devices that don't support VoLTE have to downgrade to 2G/3G for voice calls.
Even if you don't do any voice calls the phone registers itself in the network, and for doing so it needs data. Also new voice calls are signalled via mobile data. Those signaling data is not a high amount of data, but a value as you have shown in your screenshot should be reasonable for a month of VoLTE data usage.
The strange part is that the LTE voice data if you do a voice call is typically not counted as "mobile data".
Typically this behavior is making big problems when you are abroad in a roaming network with extra charges for roaming mobile data, because the mobile network providers seem to be unable to correctly mark this VoLTE signalling data as VoLTE data for which you are not charged. Instead the say "you have transmitted data via mobile roaming" now pay us the base fee for using mobile roaming data.
A second use case where data is is always use are MMS. This is the way it works for mobile generations.
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