Signal absolutely could do better in minimizing metadata by simply not requiring a phone number. Despite this obvious, huge, and dangerous shortcoming, I have never seen a single explanation of why Signal needs a phone number for signup.
Often this answer is accompanied by pure sarcasm where if you are concerned about this feature, you are told that Signal is not for you and "you can go play at being a spy and sharing a secret decoder ring with your friends", as these people regard PGP to be. I wish those Signal advocates could lay off the sarcasm, it just makes the project look bad.
[1] https://www.signal.org/blog/private-contact-discovery/
Of course, this feature is Android only as iOS doesn’t allow default alternatives.