You just create an account with the phone number as the unique ID in the database and allow every other field to be nullable, except perhaps the messages field

You just create an account with the phone number as the unique ID in the database and allow every other field to be nullable, except perhaps the messages field as you only want to create this ghost account if a message is to be sent. Possibly best to have some logic that allows an account to only have nullable fields if it is created by an existing user signing someone else up.