Instagram can be used with only a login and email, so you can be anonymous without any contact info.

If you use the Instagram app with live cell service then it's already copied all of your contact info from scanning phone details.

That doesn't mean you might as well register your number with Whatsapp, in the same way that someone can use Instagram without ever using Facebook or Messenger.

I do agree about try to use Signal in place of Whaysapp, nit along side Whatsapp, if for no other reason than to compartmentalize your data and your info. Think of it this way, seperating security from privacy, Whatsapp is far better than plain texting for security. If you want privacy and unrecoverable conversations, it starts at Signal, but still goes higher from there if a person is open to learning more technical apps like SimpleX.