All communication between WhatsApp clients and WhatsApp servers is layered within a separate encrypted channel. [This method] encrypts metadata to hide it from unauthorized network observers. No information about the connecting user’s identity is revealed.
Na FAQ do WhatsApp ,
We ask for phone numbers because that's how WhatsApp identifies WhatsApp users, making it easier for you to message friends and family quickly.
Portanto, parece que o WhatsApp usa seu número de telefone em um nome de usuário, mas esse identificador ainda é criptografado, independentemente do canal de comunicação usado (4G / LTE / WiFi).