code: public Phonenumber.PhoneNumber getUnderlyingPhoneNumber() {
          
      	Phonenumber.PhoneNumber copy;
      	
          StringBuilder e164Builder = new StringBuilder();
          PHONE_NUMBER_UTIL.format(number, PhoneNumberFormat.E164, e164Builder);
          
      	try {
              copy = PHONE_NUMBER_UTIL.parse(e164Builder.toString(), "GB");
          } catch (NumberParseException e) {
              throw new PhoneNumberParseException(
                      EX_PARSE_MSG_PREFIX + number.getNationalNumber() +"}", e);
          }
      	
      	if (number.hasExtension()) {
      		copy.setExtension(number.getExtension());
      	}
      	return copy;
      }
docs: Returns the underlying LibPhoneNumber {@link Phonenumber.PhoneNumber} instance.
  To preserve the immutability of E164PhoneNumber, a copy is made.
  @return The underlying PhoneNumber instance