Back in the 1990s Black Marble we were involved in the development of an SDK for the BT Callscape 100, a serial port based Computer Telephony Intergration product. This SDK allowed Windows developers to make use of the CTI functionality of this BT product from within any application development environment that supported Active-X controls.
The Callscape 100 has long since been discontinued by BT, but it seems that there are some people still using them for a variety of specialist purposes (but this must be getting harder as the product was designed for Window's 16bit and a serial port, neither too common on current PCs!)
In January of 2002 BT gave permission for the source code of our SDK and all the supporting information we had to be made available free to any interested parties. This information used to be on our main web site, but was removed during a recent update.