Callback emulates Windows callback functions using SAL function calls. Using Callback you can obtain a function pointer which can be used for SDK function calls (for callbacks, hooks or window procedures).

Callback for all TD versions from 1.1x up to 4.1 (2006) callback2006.zip
Callback for TD 5.1 (Unicode, please read the readme.txt) callback2007.zip

This file also contains sample applications (BrowseForFolder) and C sourcecode.
Find an ancient CenturaPro article about Callback here.

TAPI sample application using Callback tapi_callback 2003.zip
Find an ancient CenturaPro article about TAPI callback here.