You personally my feel that the phone number makes a great user ID, but I'd venture you're old enough to remember knowing people's phone numbers. I am too, but I couldn't tell you any of the phone numbers that I interact with on a daily basis today. In a few more generations, phone numbers may be even less understood by the younger users.
Usernames are nice, but you probably don't know whether your dentist uses Discord, or what is their username if they do. With Signal, however, you would probably notice (or be informed) when they sign up.