The primary focus of the Windows Telephony API is to provide useful telephony functions to desktop applications. In its simplest form, the view of telephony provided to the application developer roughly corresponds to one where telephone lines and phone sets logically terminate at the Telephony API. The physical connection may be made at the desktop, or at a LAN-based host or server, where a LAN protocol "extends" the connection of the phone lines or phone to the desktop client API.
The API provides a first party call control model on the logically terminated line as well as control of the associated phone device. First party call control implies that the application controlling the call through the API acts as if it is an endpoint of the calls it controls. This is in contrast to third party call control where the controlling application does not act as if it is an endpoint of the calls it controls. For example, with first party call control, an application can only make or answer a call if it acts as either the originating party or the terminating party of the call. A third party call control model would allow an application to establish or answer a call between "any" two parties, the application not acting as either of these parties.
This does not mean that the Windows Telephony API cannot be useful in providing call control in a third party environment. Note the usage of the phrase "act as" in the paragraph above. Each individual station on the switch is identified by its primary phone number (or other means). A possible implementation of the API's 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. The application can make and answer calls selecting any one of the addresses on the line device as the origination number. Although the application 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.