The caller ID of Clever Dialer is based on public directories. If we cannot find a phone number in one of our sources, we cannot identify the caller.
Another reason Clever Dialer cannot identify an incoming call might be that the caller has deliberately suppressed his phone number. The phone then shows information like “private” or “unknown number”. In these cases it is technically impossible to get the phone number and identify the caller. No app can identify suppressed numbers.