If you modified admin/orders.php to read
<tr>
<td class="main"><strong><?php echo ENTRY_TELEPHONE_NUMBER; ?></strong></td>
<td class="main"><?php echo $order->customer['telephone']; ?></td>
</tr>
<tr>
<td class="main"><strong><?php echo ENTRY_FAX_NUMBER; ?></strong></td>
<td class="main"><?php echo $order->customer['customers_fax']; ?></td>
</tr>
then you will just get something like this on the orders page:
Telephone Number: 09-8765 4321
Fax Number: <BLANK>
E-Mail Address: [email protected]
with a <BLANK> field for the actual fax number simply because the fax number is not stored in the orders table.
So me thinks you either need to pull the fax number from the customers table or modify the ZC code in a way that the fax number is stored in the orders table when an order is created.