Usernames are coming in 2021. As of now, it's not necessary to use your actual mobile number to signup though. It's well possible to use a virtual number like Google Voice, and even landline numbers work! By "other non-phone devices", were you referring to Signal on front-end-less platforms like Raspberry Pi CLI?
Always found it impossible to use Google Voice outside the US.
They have already announced that they are working on this and it will arrive in 2021.