Oppo
Set a Default Calling SIM for a Contact auf Oppo
On a Dual SIM Oppo phone running Coloros, the calling SIM used for a contact usually falls back to whatever SIM you dialed with last, unless you lock it to a specific card.
Coloros lets you attach a default calling SIM directly to a contact entry, so every call to that person goes out through the SIM you choose, no matter which SIM was active before.
To set it, open Contacts ➔ Choose a Contact ➔ Open the Menu ➔ Tap on Set calling SIM ➔ Choose a SIM card and tap OK.
This setting is stored per contact, not globally, so you can mix SIM assignments across your address book without changing the default SIM for the whole device.
If a contact has multiple numbers saved, for example a private and a business number, check which number you are dialing, since the assignment applies to the contact entry rather than to a single number.
If you want to change which SIM handles calls by default across the whole phone instead, adjust that under the general network settings rather than editing individual contacts.