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Correct. The only people who can read a message (or hear a voice call) are their intended recipients.

Would it be possible for Facebook to introduce a backdoor later without breaking the Signal protocol, or alternatively "forking" it while keeping compatibility between their clients?

The client has access messages in clear text. A backdoor could easily deliver those messages to a third-party, yes.

This is not something E2E protocols can protect you from. You'd have to audit every piece of firmware and software on your device to verify that's not happening.

If he does not answer does it mean whatsapp or signal servers can see the message?

This is the first thing that came to mind when I read this, having clarification from them would be reassuring.