def to_s(format = '')
      return '' if self.empty?

      case format
      when Symbol
        format = @@formats[format]
      when ''
        format += "(%a) " unless @area_code.nil? or @area_code.empty?
        format += "%p-" unless @prefix.nil? or @prefix.empty?
        format += "%n" unless @number.nil? or @number.empty?
        format += " x%e" unless @extension.nil? or @extension.empty?
      end

      format_number(format)
    end
def to_s(format = '')
      return '' if self.empty?

      case format
      when Symbol
        format = @@formats[format]
      when ''
        format += "(%a) " unless @area_code.nil? or @area_code.empty?
        format += "%p-" unless @prefix.nil? or @prefix.empty?
        format += "%n" unless @number.nil? or @number.empty?
        format += " x%e" unless @extension.nil? or @extension.empty?
      end

      format_number(format)
    end
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