When you add new contacts, the default order for phone numbers is:
Business | Home | Business Fax | Mobile
Is there a way to change what these defaults are?
No, I don't think there is a way to change the default order. However you can change the phone number type using the pick list.
You can do it by customizing Contact's form. Tutorial shows the process for Outlook 2003, but for 2007 it's pretty much the same.