Supplementary Telephony includes features found on modern PBXs, such as hold, transfer, conference, park, and so on. All supplementary features are optional.
A service provider offers any combination of the services. A service provider specifies which services it supports through responses to functions such as TSPI_lineGetDevCaps or TSPI_lineGetAddressCaps. A single supplementary service can consist of multiple function calls and messages.
Note TAPI, and not the service provider developer, defines the behavior of each of these supplementary features.
A service provider should provide a supplementary telephony service only if it can implement the exact meaning as defined by TAPI. If not, the feature should be provided as an Extended Telephony service.
All phone-device services are part of Supplementary Telephony.