In Win32 Telephony, supplementary functions are functions whose form and functionality have been defined by the API description, but which are not required in Basic Telephony. Developers of telephony applications and service providers may choose to implement these functions to suit the design of their custom products. That is, in contrast to Basic Telephony functions, Supplementary Telephony functions are optional.
This section describes the Supplementary functions used with TAPI-defined line devices. For convenience, they are grouped into general functional categories.