You can set up call forwarding on the Oppo smartphone, and the phone settings offer call forwarding, If busy, If unanswered, If not available and Always forward.

Android 10 (ColorOS 7.0)
Step 1:  Open the  Phone -App
Step 2:  Open the  Menu
Step 3:  Tap on  Settings
Step 4:  Tap on  Carrier call Settings
Step 5:  Tap on  Call Fowarding
Step 6:  Choose  SIM-Card  you want
Step 7:  Choose between  Always Forward,   When Busy ,  When Unanswered  and  When Unreachable
Step 8:  Enter a  Phone numer  and tap on  On
  1. Open the Phone-App
  2. Open the Menu
  3. Tap on Settings
  4. Tap on Carrier call Settings
  5. Tap on Call Fowarding
  6. Choose SIM-Card you want
  7. Choose between Always Forward, When BusyWhen Unanswered and When Unreachable
  8. Enter a Phone numer and tap on On

Oppo Instructions
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