Some callers deliberately suppress their telephone number and thus do not transmit it. The display then says, for example, “unknown number” or “private”. In this case, it is technically impossible to display the phone number. Other apps cannot do this either. Some app provider deceive their users into believing that they can display suppressed phone numbers. However, we want to be honest and therefore explicitly point this out in our app description in the Play Store and in our product tour.
If you want to know more about how number suppression works and why no one can access the number, you can find more information in this Wikipedia article.