If we receive a call at an inconvenient moment, such as a meeting, we can reject the caller with a short SMS message and inform them that we will call back in a few minutes or later, for example.

After selecting the desired message, the caller is immediately rejected and an appropriate SMS message is automatically sent from the OnePlus smartphone.

Android 10 | OxygenOS 10.0
Step 1:  Tap on  Reply
Step 2:  Choose a  quick response  to reject the call and send a message
  1. Tap on Reply
  2. Choose a quick response to reject the call and send a message

OnePlus Instructions