More: * "A mobile phone knows its phone number." The phone cannot know its own phone number without making a call or sending a text message. Some SIM cards car

More: * "A mobile phone knows its phone number." The phone cannot know its own phone number without making a call or sending a text message. Some SIM cards carry an "own phone number" record but it is not authoritative and sometimes inaccurate. * "A SIM card is permanently assigned to a phone number." SIM cards to phone number relationship can be many:many and change over time. * "<Social app X> requires phone number to sign up, so we should too." Users actually react very differently when a messaging app asks for their phone number (useful to find contacts) vs a calculator app or a game or an app where they want to be anonymous.