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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def numbers(self):
 """ Access the numbers :returns: twilio.rest.pricing.v1.voice.number.NumberList :rtype: twilio.rest.pricing.v1.voice.number.NumberList """

        if self._numbers is None:
            self._numbers = NumberList(self._version, )
        return self._numbers