If someone calls you, while you are already phoning with someone else, your Huawei device will notify you optically and acustically that another call is incoming. You can turn on or off this little knocking sound in the settings.

Note: If you are using dual SIM cards, you can activate or deactivate the Call waiting function for each SIM individually.

Android 10 | EMUI 10.0
Step 1:  Open the  Phone app
Step 2:  Open the  menu
Step 3:  Open the  settings
Step 4:  Tap on  More
Step 5:  Activate or deactivate  Call waiting
  1. Open the Phone app
  2. Open the menu
  3. Open the settings
  4. Tap on More
  5. Activate or deactivate Call waiting

Huawei Instructions