You have to use a phone number to create a Mixin account.
To register a number.
- Install and open the latest update of Mixin Messenger on your Android or iOS device.
- Enter and confirm the registered number
- Wait for the SMS verification code to arrive.
- Input the SMS verification code then you get a Mixin account
Note: Changing your SIM card doesn't automatically change your Mixin Messenger login number; you have to change it manually.