Online scams are increasingly common and you must always be very careful while surfing the net: in particular, it would seem that a particular practice is becoming common on WhatsApp.
According to what was reported by WABetaInfo , some users have received messages from VOIP numbers (which among other things should be banned on the messaging platform): these messages read phrases like “Sorry, who are you? I found you in my address book” and, continuing the conversation, the scammer starts asking for personal information and social contacts.
Needless to say, it is extremely discouraged to reply or provide information that could be used for less than noble purposes: the advice is to report the contact to WhatsApp as soon as possible (you can do it from the screen that contains all the contact info) and block the number.