Ultra Hal should work with Cepstral voices, there is even a link to Cepstral on the main page of Zabaware, I bought mine via the Zabaware site.
The Cepstral voices should simply be a selected option in Options/Speech in Ultra Hal, for both registered and unregistered Cepstral voices. Have you actually been able to use the Callie voice in any other applications? Have you heard it speak on your computer? If not, the file could be corrupted.
Cepstral has a speech utility called Swifttalker that works with Cepstral voices. I don't recall wether it comes with unregistered voices or not, but if it did, can you use it to hear Callie speak?
I think Cepstral voices require SAPI5, not 4. I would guess that Vista has that, but I don't have Vista, so I don't know if there is somewhere you can check to see if it's set to use SAPI4 or 5.
Be sure that Vista has given the Cepstral program permission to operate with other programs. Make sure the Cepstral download was good, try it out on another computer if you can. Other than that, I'm out of suggestions.
- Chaz