TAPI uses a first-party call control model on the logically terminated line, as well as control of the associated phone device, if any. Service providers must allow applications to control telephone calls as though they were an endpoint (the initiator or the recipient) of the call.
The application can make calls; be notified about inbound calls; answer inbound calls; invoke switch features such as hold, transfer, conference, pickup, and park; and can detect and generate DTMF (Dual Tone Multi-Frequency) tones for signaling remote equipment. An application can also use TAPI functions to monitor call-related activities occurring in the system.
In contrast, third-party call control means that the controlling application does not act as an endpoint of the call. A third-party call control model allows an application to establish or answer a call between any two parties; the application does not act as either of these parties.
A service provider can treat a set of stations on the switch as a single line device to which multiple phone numbers are assigned. Each phone number on the line device maps to one of the stations on the switch. The application can answer calls or make calls, selecting any one of the addresses on the line device as the origination number. Although the application appears to be the originating party, a call is actually established between the station whose address was selected by its originating number and the other party. However, this implementation is a type of third-party call control and is not a design goal of TAPI, which emphasizes first-party call control applications.
Each individual station on the switch is identified by its primary phone number (or other means). A possible implementation of the line and phone behavior by a service provider is to model the collection of all stations on the switch as a single line device that has multiple phone numbers assigned to it; each phone number on the line device, in reality, maps to one of the stations on the switch.
TAPI can make and answer calls, selecting any one of the addresses on the line device as the origination number.
Although TAPI appears to be the originating party, in reality, a call will be established between the station whose address was selected by its originating number and the other party.
A distributed service provider supports calls shared by multiple computers in a local area network by communicating with service providers on the other computers.