OnePlus
Make a Phone Call with the Phone App auf OnePlus
Placing a call on a OnePlus phone runs through the pre-installed Phone app, but the flow changes the moment two SIM cards are active. Instead of connecting right away, the system can pause to ask which line should handle the call, and that extra step is where most confusion starts.
Open Open the Phone app and choose either Contacts or Dial pad. Enter a number or tap a saved contact, then confirm with Tap on the green phone icon. On a single-SIM setup the call connects immediately. On a Dual SIM device, a prompt appears so you can Choose a SIM card (Dual SIM) for that specific call.
Why the SIM prompt shows up
OxygenOS tracks call preferences per contact and per SIM slot. If no default line is set for a number, the app asks every time instead of guessing, which protects against a call - and the cost that comes with it - leaving through the wrong carrier.
If the prompt never appears and calls always leave through one SIM, that number likely already has a fixed default assigned. Removing that default brings the choice screen back on the next call.