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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 incoming_phone_numbers(self):
 """ Access the incoming_phone_numbers :returns: twilio.rest.api.v2010.account.incoming_phone_number.IncomingPhoneNumberList :rtype: twilio.rest.api.v2010.account.incoming_phone_number.IncomingPhoneNumberList """

        if self._incoming_phone_numbers is None:
            self._incoming_phone_numbers = IncomingPhoneNumberList(
                self._version,
                account_sid=self._solution['sid'],
            )
        return self._incoming_phone_numbers