Microsoft Knowledge Base Article 259375 contains the following summary:
Windows 2000 Server and Windows 2000 Advanced Server include a telephony server for distributed access to shared telephony devices. For example, a LAN-based server may have multiple connections to a local telephone switch or PBX. Telephony Application Programming Interface (TAPI) operations invoked by a client are forwarded over the network to the server. The server can then use a third-party call control between itself and the PBX to fulfill the client's call control requests.