Someone knows all my family numbers & have been sending adult emojis & content

So long story short someone has been harassing us and the reason I want to investigate is the person have both my wife & my numbers. We have a total of 3 WhatsApp’s where this person is sending garbage. I know I could have simply blocked which we always do but the alarming situation is how do he have contact of all of us numbers & whatsapp. Would someone be kind enough to help us investigate. I can share the persons contact number