Some Contacts Happen to find my New WhatsApp Number somehow

Hi All,

Reference to my Title subject, How is it possible for some of my contacts (my in-laws as am not going along well with them) to know my new mobile number (until now 4 of them which I have not shared with anyone) whenever I use it with a newly installed WhatsApp every time even after I do factory reset my new Android phone and now after I installed Anti Virus also. As soon as I come online on newly installed WhatsApp with my new mobile number they come to know about it and then they toggle around online-offline. Is it through by means of :-

Hacking software (Though For them to get my online Intimation, they should definitely know my new number)

Anyone known to them within the Internet Service Provider Office by Hacking my WiFi connection somehow or my IP or Mac Address

Anyone known to them within the Cyber cell Office

Regards
DeJo