OnePlus
Assign a Default SIM Card to a Contact auf OnePlus
On a dual SIM OnePlus phone, each contact can be linked to a specific SIM for outgoing calls. Without this link, Oxygen OS asks which SIM to use every single time you dial that contact.
The setting is not part of the general dual SIM menu. It sits inside the individual contact card, so the choice only has to be made once per contact.
To assign it, follow this menu path: Open the Contacts app ➔ Choose a contact ➔ Open the menu ➔ Tap on Set calling SIM ➔ Choose a SIM card ➔ Confirm with OK.
After confirming, every call you place to that contact automatically goes through the selected SIM. Contacts left without an assignment still trigger the usual SIM selection prompt before each call.
This setting only controls voice calls. Messaging apps and mobile data keep their own separate SIM preferences, so check those individually if you also want fixed defaults there.