When dialling a telephone number on the keypad, the dial key tones can be played if desired. By default, the key tones are deactivated when dialling on the Oppo-Smartphone. If you want the tones, you can activate them in the settings.

Android 10 | ColorOS 7.0
Step 1:  Open the  Settings
Step 2:  Tap on  Sound & vibration
Step 3:  Enable or Disable  Dial pad tone
  1. Open the Settings
  2. Tap on Sound & vibration
  3. Enable or Disable Dial pad tone

Oppo Instructions