> Some people do not own phones, or do not wish to provide you with their telephone number when asked. Do not require a user to provide a phone number unless it is essential, and whenever possible try and provide a fallback to accommodate these users.
Signal, WhatsApp and Telegram are spectacular design failures on this point because they assume that every person has a phone number and also that every person has their own private (non-shared) and unique phone number. Facebook, Google, etc., require a phone number for verification and believe that it’s sufficiently adequate to thwart spammers.
The whole “must enter a phone number” phenomenon is a big mess, introduces privacy issues and excludes many people. None of the companies mentioned above would agree that excluding people is a goal for them, but they’ve made it so.