There are several benefits to moving your business logic and data access code out of ASP pages and into COM-based DLLs. Code is easier to maintain, you can use a multitier design, and your app can exploit the integration between IIS and the MTS runtime.
Windows NT 5.0 will support TAPI 3.0, but to exploit that down the road you'll need to understand TAPI 1.x and 2.0 now. Operators are standing by to give you all the TAPI information you need, and explain how Unimodem will provide dialing assistance.
You can make IIS handle HTTP requests in almost any way you want. With an ISAPI filter, you can modify how HTTP requests are processed by the server. And a server-side script interpreter can customize how certain types of files will be used.