module_class_methods.rb

module Formatter
   def self.included base
    base.extend ClassMethods
  end
  
  module ClassMethods
    def class_method
      "yahy"
    end
  end

  def formatted_phone
    phone.delete("^0-9")
  end
end

class Payment
  include Formatter
  
  def phone(phone)
    formatted_phone(phone)
  end
end

class Contact
  include Formatter
  
  def phone(phone)
    formatted_phone(phone)
  end
end