Are you saving the CSV as an Excel file?

Have you tried using the canned format for phone numbers? The canned format in Excel might not suit your numbers as it may be for US which in scientific notation would typically have an exponent of 9 or 10 (depending on whether the country code is included). I presume you are in New Zealand so am unsure why any of your numbers would have an exponent of 11. At any rate, you should be able to devise a custom format to suit your needs.

If you would please attach a sample sheet, it may help.