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