Would invites be a solution? Anyone can sign up if they provide a number, otherwise you need an invite from someone with a number linked. It would clump the identity/legitimacy for all invitees into origin number, but still allow disparate accounts.
Sure, I don't mind if they ask for my phone number if they think that's a better default onboarding flow, but allow users to bypass it.
With all that said, I don't think it's really only about user friction.
What is required at the moment is any phone number, not your phone number. You can use a phone booth even.