s/user/phone number/

Not at all, a user is a phone number.

Do you have a recommendation on how they would prevent fraud and abuse w/o using a phone number while also maintaining the same level of low friction?

What fraud and abuse? They can prevent fraud and abuse by doing anything, including by doing nothing, because those aren't concepts that apply to their product.

Spam is one kind of abuse.

So? What's the threat model? How does having phone numbers help?

You're making assertions about what Signal needs, and you're doing it without knowing their threat model.