I’m drying.
Searched for “You must have at least one SIM slot enabled” (pops up when one tries to disable both SIM slots). But as error messages, they are stored elsewhere than in the searched files.
Maybe something interesting here?:
/usr/lib/qt5/qml/Sailfish/Settings/Networking/SimDependentSettingsToggle.qml
/usr/lib/qt5/qml/Sailfish/Telephony/SimErrorState.qml