Signal uses phone numbers as (the only) unique identifier in their system currently so SMS (or phone call) is necessary to verify the device owns the number. T

Signal uses phone numbers as (the only) unique identifier in their system currently so SMS (or phone call) is necessary to verify the device owns the number. They've been talking about moving away from phone numbers as identifiers for a while and have implemented features like account pins that head in that direction but it hasn't happened yet.