It's guy that promised you private chats by asking you to register with a phone number.
Privacy and anonymity are not the same thing. Signal foregoes the latter to provide the former to a much wider range of people; foregoing the latter enables substantially better usability for non-technical people.
This makes no sense. If you wanted to prove that you would need to have a signal equivalent that does not use phone numbers. Where is that?
Anyway here’s some alternatives:
Briar (no iOS client?)
Threema (not free as in free beer)
Wire
Matrix
XMPP
Telegram (can be used E2EE for 1:1 chats only and is off by default)