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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