Ascendis Caller ID supports the voices included with Windows XP, Windows Vista, and Windows 7. It also supports compatible voices, or those compatible with SAPI5.
Additional voices, including demos and samples, are available on the Ascendis Software LLC website:
http://www.ascendis.com/callerid/voices.php.
Windows XP comes with a single voice ("Microsoft Sam") for text-to-speech. It's not great, but it works. Windows Vista and Windows 7 come with "Microsoft Anna", which is better.
Technical Details
Ascendis Caller ID uses Microsoft's Speech API (SAPI5) for speech synthesis.