Xiaomi
Turn a SIM Card On or Off auf Xiaomi
On dual SIM Xiaomi phones both SIM slots stay active by default, so calls, texts and mobile data run through whichever card you last used for that purpose. Switching a SIM off in MIUI is different from removing the physical card: network registration for that number pauses, so no calls or messages reach it and it disappears from the SIM selection prompts.
This matters when one line is a work number you only need during office hours, or when a prepaid SIM is running low on data and keeps interrupting apps with roaming charges. Turning that SIM off keeps it in the tray but removes it from active use until you toggle it back on.
To switch a SIM card, open Tap on Settings ➔ Tap on SIM cards & mobile networks ➔ Choose a SIM card (Dual SIM) ➔ Turn On or Off the selected SIM card ➔ Tap on OK. The setting applies instantly and survives a restart, so you do not need to repeat it after rebooting the phone.
Disabling a SIM does not affect the other slot: calls, texts and data on the remaining card continue normally. If you rely on that number for two-factor authentication codes, disable it only when you are certain you will not need incoming messages during that time.