example.rb

class Parent
  def initialize(name)
    @name = name
    @phone = build_phone_number
  end
  
  def build_phone_number
    raise 'Subclasses must override build_phone_number method!'
  end
end

class Child < Parent
  def initialize()
    super("Tom Bombadil")
  end
  
  def build_phone_number
    "1234567890"
  end
end