Try this on a Windows Mobile 6 Smartphone or Pocket PC (Standard or Professional Editions). Bring up your Call History list. Navigate to a call to/from a person who you know to have multiple phone numbers in your Contacts list. Then press the Left or Right navigation. It should rotate through the phone numbers for that contact. You can settle on one of the alternate numbers and call it instead of the number originally called from.

It worked on WM6 devices but did not seem to work on a WM5 Pocket PC Phone Edition.