address_book.rb

class AddressBook
  attr_accessor :contacts

  def initialize
    @contacts = {}
  end

  def numbers *names
    names.map do |name|
      @contacts[name]
    end.flatten
  end

  def register_contact name, numbers
    @contacts[name] = numbers
  end

  def self.default_book
    return @book if defined? @book
    book = new
    book.register_contact :all, all_mobile_numbers
    book.register_contact :all_staff, all_staff_numbers
    book.register_contact :customer1, customer1_numbers
    book.register_contact :customer2, customer2_numbers
    @book = book
  end
end

AddressBook.default_book