Using phone numbers as identifiers for encrypted messages is the core feature of Signal. It was marketed from day one as a drop in SMS replacement. Initially it

Using phone numbers as identifiers for encrypted messages is the core feature of Signal. It was marketed from day one as a drop in SMS replacement. Initially it even used SMS as the transport for encrypted messages. It was literally called "TextSecure". This is why I have always found the attacks on it using phone numbers to be amusing.