No idea about how mobile phone number usage and ownership is done in Australia (where the Session team is based) but for example in Poland and probably in most

No idea about how mobile phone number usage and ownership is done in Australia (where the Session team is based) but for example in Poland and probably in most European countries is impossible anymore to use a SIM card without registering it first under name and last name. And then, phone number is also used in state e-services where it is required to provide it - either directly or via third-party bank logins. So, perhaps for obscure everyday apps use it might be possible to avoid being tied with your phone number but in some other places is rather unlikely.