On your smartphone, an incoming call usually shows the number of the caller, or the contact name, if it exists in your contact list. Now it can also be that the display shows "Private" or "Unknown".
Here you will now legitimately ask the question: What does it mean when a call comes in with status "private"?
The answer to this question will be given to you here. If the caller's telephone number is not shown on the display, instead of "Private" or "Unknown", this means that the caller has activated the suppression function. The call number suppression or the hiding of the own telephone number can be activated at your mobile carrier.
You usually go into the system settings of your smartphone and then select the option "Do not show phone number". Here is an example:
Unfortunately, you cannot recognize with which number the unknown calls. But you can simply decide not to accept the call or block it by default. For example, a so-called blacklist can be used.