We can also block contacts stored on the OnePlus smartphone using the Contacts app and report them as spam if desired.

Once a contact is blocked, no calls or SMS messages from that phone number will be allowed, transferred or received.

Note: To unblock a contact, we have to follow the same steps.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Contacts app
Step 2:  Choose a  contact
Step 3:  Open the  menu
Step 4:  Tap on  Block numbers
Step 5:  Confirm with  Block
  1. Open the Contacts app
  2. Choose a contact
  3. Open the menu
  4. Tap on Block numbers
  5. Confirm with Block

OnePlus Instructions