OnePlus
Enable or Disable Call Forwarding auf OnePlus
Call forwarding on a OnePlus phone is not a single on/off switch. It is a carrier feature with several conditions, and OxygenOS only gives you the menu to configure whatever your network operator allows. That is why the available options can differ between two OnePlus phones on different carriers.
The setting lives per SIM card. On a dual SIM device, open Phone app ➔ Menu ➔ Settings ➔ Calling accounts and pick the SIM you want to configure before tapping Call forwarding. Choosing the wrong SIM here is the most common reason forwarding seems to do nothing.
Choosing the right condition
Inside Call forwarding you pick when calls should redirect, for example when the line is busy, unreachable, or unanswered. Enter the destination number to activate that condition, or tap Turn off to cancel it.
If calls only fail in areas with weak signal, forwarding on unreachable is the setting to check first. If you want every call sent elsewhere regardless of status, forwarding on all calls covers that, but remember it also blocks calls from reaching your OnePlus phone entirely while active.
Forwarding depends on network support. If changes do not save or an error appears, the carrier may not offer that particular condition, and no setting on the phone will fix that.