If we are in the middle of a phone call with the OnePlus smartphone while a second call is coming in, we can be notified via the call waiting function.

A signal sounds to alert us to the other call, which also provides further options:

Note: When using Dual SIM, the call waiting feature can be set individually for each SIM card.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Phone app
Step 2:  Open the  menu
Step 3:  Open the  Settings
Step 4:  Tap on  Calling accounts
Step 5:  Choose a  SIM card   (Dual SIM)
Step 6:  Tap on  Additional settings
Step 7:  Enable or disable  Call waiting
  1. Open the Phone app
  2. Open the menu
  3. Open the Settings
  4. Tap on Calling accounts
  5. Choose a SIM card (Dual SIM)
  6. Tap on Additional settings
  7. Enable or disable Call waiting

OnePlus Instructions