TAPI version 2.0 added a small number of enhancements to the basic TAPI 1.4 functionality. However, there were some major architectural changes that greatly improved its stability. Most of the changes were fundamental changes necessary to bring TAPI to Windows NT and take advantage of Windows NT features (full Win32 support, services, Unicode). However, these changes were internal to TAPI and had little effect on TAPI applications.
TAPI 1.3 and 1.4 applications (Win16 applications by way of a thunking layer) work well on Windows NT 4.0. However, the effect on service-provider developers was significant. Service providers for Windows NT must be fully 32-bit Unicode DLLs that can run in the context of TAPISRV, not in the context of the TAPI application (as did all earlier Win16 TSPs). TSPs designed for TAPI 1.4 do not work on Windows NT.
TAPI 2.0 also added the important features of call center support and Quality of Service support.
The TAPI 2.0 system binaries (as part of Windows NT 4.0) support TAPI versions 1.3 and 1.4 for both Win16 and Win32 applications. However, they only support version 2.0 for Win32 applications and only support TAPI 2.0 service providers.