Some people call with suppressed phone numbers. The smartphone  shows something like "private" or "unknown number". In these cases, the caller decided against transmitting his phone number, and we have no way to access it. Clever Dialer cannot show any information about the caller. This goes for all other apps as well, even if they claim that they can access suppressed phone numbers. As we do not want to mislead our users, we point that out in the app description in the Play Store and our product tour.