Wire.com E2E encryption does not require phone number or social graph. They are also working to standardize the wire protocol via IETF MLS, with cryptographers from multiple companies.
OWS is working on foundations to allow people to use usernames and link them with FCM / push notifications without the central signal server being any wiser than currently with just phone numbers.
My hope is to be able to add ephemeral user ids so to my work folks I'm JustMe but family folks I'm FamilyMe and internet blog readers I'm MyProfessionalMe
It's probably secure against phone company snooping. But I wouldn't trust it much past that.