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 G

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?