def default_backend=(value)
      raise ArgumentError, "default_backend must be a symbol!" unless value.is_a? Symbol

      unless @backends.keys.include? value
        raise ArgumentError, "Unregistered backend cannot be set as default!"
      end

      @default_backend = value
    end
def default_backend=(value)
      raise ArgumentError, "default_backend must be a symbol!" unless value.is_a? Symbol

      unless @backends.keys.include? value
        raise ArgumentError, "Unregistered backend cannot be set as default!"
      end

      @default_backend = value
    end
Specify default sms backend. It must be registered.

 @param value [Symbol] Backend key which will be used as default