Each phone number box has a drop-down arrow next to it that you can use to
display additional numbers.

Outlook has provision for some 19 different built-in phone numbers if memory
serves correctly, taking into account fax , mobile, car, radio, telex, ISDN,
TTYTDD numbers and the like. Any one of the 4 spots that appear on the form
itself can be assigned to any of those numbers on a per contact basis
(changing which of the 4 numbers that are visible does not affect existing
numbers in any of the other fields - you just need to go to "All fields" -->
"Phone# fields" & "Fax/Other Number fields" to see the respective values).

If you want more physical phone number fields to actually appear on the form
itself - you'll have to design a custom form to accomplish that.

Karl