After turning on Use mobile data during calls, if you set the main card as the Internet card, when the secondary card receives a VoLTE call, it will temporarily switch to the secondary card to access the Internet to ensure that the network is still smooth during the call and switch back to the main card when the call ends.
Setting path:
realme UI 3.0 & above versions: Settings > Mobile network > Advanced/More settings > turn on Use mobile data during calls
realme UI 2.0 & 1.0: Settings > SIM card & mobile data > turn on Use Internet during calls on secondary data SIM
realme UI R & Go & S & T & U Edition: Not supported
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