In Office 2007 I knew how to set up the phone/fax/cell numbers so you only had to enter the area code and number (e.g. 8005551212) and when you hit Enter, the number showed up as +1 (800) 555-1212. I cannot find or figure out how to do that in Office 2010. Can anyone help? Many thanks.

You will need to set your local area code first. You can configure it in the Windows dialing properties (control panel,phone and modem options)

Once set, Outlook will add the +1 prefix before dialing.