You can use Wi-Fi calling on your Android or iPhone to make calls using Wi-Fi rather than your cellular network.
Alternatively, if almost every unknown caller is a dud, you can safely pick the “Silence” option and let voicemail sort them out. This is especially true in an age where number spoofing is rife, where ...
You can make a call private by hiding your phone number in a few different ways, including using your phone's settings.