Service Levels

Applications whose telephony functionality goes beyond the most basic call control or are meant to handle incoming calls must be built using the Telephony API (not Assisted Telephony). The Telephony API defines three levels of service:

• The elementary level of service, called Basic Telephony, which provides a minimum set of functions that corresponds to Plain Old Telephone Service (POTS). TAPI service providers are required to support all Basic Telephony functions.
• The Supplementary Telephony level of service, which provides advanced switch features such as hold, transfer, and so on. All supplementary services are optional; that is, the service provider is not required to support them.
• The Extended Telephony level of service, in which the API provides well-defined API extension mechanisms that enable application developers to access service provider-specific functions not directly defined by the Telephony API.