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 11 | ColorOS 11.0
Step 1: Tap on  Reply
Step 2: Choose a  Quick response  to reject the call
  1. Tap on Reply
  2. Choose a Quick response to reject the call

Oppo Instructions