Samsung
Set Default SIM Card for Calls auf Samsung
On a dual-SIM Samsung phone, one card is usually set as the default for outgoing calls. If that default points to the wrong SIM, every call you dial starts on the card you did not intend to use, which gets expensive fast if that SIM has no plan or roaming active.
This behavior is controlled by Samsung One UI in the SIM card manager, not by the dial pad itself. Switching SIMs manually before a call only works if the setting allows it, otherwise the phone silently falls back to the stored default.
To change it, open Settings ➔ Connections ➔ SIM card manager ➔ Calls, then choose SIM 1, SIM 2, or Ask always. Picking Ask always shows a quick selection screen before every call, which is useful if you regularly choose between a work and a private number.
The same option is reachable faster from the status bar: pull it down, tap the SIM icon, and pick the card or the ask option directly. If one SIM slot is disabled or the device only detects a single active SIM, the selector for the second card will not appear here at all, since there is nothing to choose between.