If I connect to a network, is the network admin able to identify user phone numbers used by Whatsapp through the network traffic? In other words, are phone numbers used to signup by messaging apps such as Whatsapp or Telegram viewable by network admins?
When you are using Whatsapp the communication is encrypted by TLS with the webserver. So no telephonenumber can be seen unless you are able to decrypt HTTPS traffic. Network admins would only see that an IP in the network is using Whatsapp but not who will talk to whom and what will be written inside the chat. This is protected by E2E and TLS encryption. If you want to know more about the used encryption take a look into the whitepaper found here -> Link