Unity was an early adapter of Telephony Application Programming Interface (TAPI) and made use of it for both its original voice board interface and its interface into CallManager. It uses TAPI through the telephony service provider (TSP), which is the interface connected to the voice board or CallManager. The early voice board manufacturer used by Unity was Dialogic and is still in use today.
Unity's implementation into CallManager became more scalable and capable when it began using the Skinny Station Protocol in its TSP to connect to CallManager.
Along with Unity's support of multiple integration types, the Unity 4.0 offering includes a TAPI-independent implementation of the Session Initiation Protocol (SIP). In the future, the next-generation replacement of legacy voice boards used by Dialogic includes SIP as its primary interface. Unity will integrate with this interface for Dialogic. In addition, Unity can support third-party SIP proxy servers from different vendors.
A couple of very important chapters focus on these integration technologies: Chapter 6, "Components and Subsystems: Telephony Services," and "Chapter 17, "Unity Telephony Integration." In addition, you will find switch file settings in the appendix.