If we are busy and cannot answer the call, the call can be rejected with a short answer. By tapping on "Messages" the call is rejected and a popup with the short answers opens on the Oppo-Smartphone. Besides the selection which can be edited, an individual message can be written.

Android 10 (ColorOS 7.0)
Step 1:  Tap on  Message
Step 2:  Choose desired  Message
  1. Tap on Message
  2. Choose desired Message

Oppo Instructions