If we are busy, we can reject an incoming call with a short answer, the call is automatically rejected and the Samsung device automatically sends the selected message as an SMS to the caller.

The standard short replies (quick messages) can also be edited.

Android 10 | One UI 2.0
Step 1:  Wipe  Send message  up
Step 2:  Choose desired  message
  1. Wipe Send message up
  2. Choose desired message

Samsung Instructions