Xiaomi
Set Default SIM Card for Outgoing Calls auf Xiaomi
Xiaomi phones with two active SIM slots ask which card to use before every outgoing call unless you tell the system otherwise. This confirmation dialog comes from the dual-SIM manager built into MIUI, which treats both cards as equal until you pick a default.
Setting a default SIM card skips that dialog for regular phone calls. The change only affects voice calls, not mobile data or text messages, which each keep their own separate default setting.
To set it, go to Settings ➔ SIM cards & mobile networks ➔ Select SIM card to dial and choose SIM 1 or SIM 2 instead of leaving it on the prompt option.
If you rely on both numbers regularly, for example one private and one business line, leaving the selection dialog active can still make sense. It works as a reminder to pick the right number before dialing, especially if you tend to forget which SIM is active.
Once a default is set, the same SIM handles every outgoing call automatically unless you manually switch it while dialing. You can return to the same menu at any time and reset the option back to ask every time.