paypal standard does not offer a way of accepting the phone number, this is a limitation to the paypal standard system.
you could pass the phone number into another field though such as the Address 2 field.
on the confirm page, find the following code:
<input type="hidden" name="address2" id="address2" value="<?php echo((isset($_POST["shipping_street2"]))?$_POST["shipping_street2"]:"") ?>" />
and change it to:
<input type="hidden" name="address2" id="address2" value="<?php echo((isset($_POST["shipping_street2"]))?$_POST["shipping_street2"]:"") ?>-<?php echo((isset($_POST["shipping_phone"]))?$_POST["shipping_phone"]:"") ?>" />