Realme
Set Default SIM Card for Calls auf Realme
On a dual-SIM Realme phone, the calling app checks a setting called Calling under the SIM management menu before every outgoing call. If that setting is on Always ask, you get a pop-up asking which SIM to use every single time you dial a number.
Switching this to a fixed SIM removes the pop-up completely. The system then routes every call through the chosen line automatically, no matter which app you dial from.
You can change this anytime under Settings ➔ SIM card & mobile data ➔ Calling ➔ choose SIM 1, SIM 2, or Always ask.
This setting only controls voice calls. Mobile data and SMS have their own default SIM options in the same menu, so changing calls does not touch your data connection.
Keep Always ask instead if you regularly use different SIMs for different contacts or carriers, for example one line for work calls and another for personal ones. Setting a fixed default only makes sense if you consistently use one SIM for outgoing calls and rarely need the second one for that purpose.
If the default SIM loses signal or is set to airplane mode, calls through that line will simply fail instead of falling back to the other SIM automatically.