Each line device is assigned one or more addresses. An address corresponds to a telephone directory number, and it is actually assigned twice: first, by the telephone company at the switch, and second, by the user while configuring the local system. If a telephone number is changed at the switch, the user will normally need to assign the new number at the local system, although some systems may be sophisticated enough to perform the reassignment without human interaction.
After addresses have been assigned to lines, the service provider assigns address identifiers to addresses. An address identifier is a number between 0 and the number of addresses on the line minus one. Because each address depends on its line for its existence, the address's identifier is meaningful only in the context of the associated line device. For this reason, an address name consists of not only the address identifier, but also an identifier of the line.
More than one address can be assigned to a line device. (The number of assigned addresses is returned by the TSPI_lineGetDevCaps function.) These address-to-line assignments (as modeled by the Telephony SPI) are static in that they change only when reconfigured in any of several ways at the switch or network. The relationship between an address and a line and between the address and other local addresses is known as the address's configuration.
Usually, one of the addresses assigned to a line is that line's primary address. A primary address is the one address on a line that can uniquely identify the line device. This primary address may also be assigned to other lines as a nonprimary address.
The network or switch can configure address-to-line assignments in several ways. Usually, one of the addresses assigned to a line is that line's primary address. A line's primary address is able to uniquely identify the line device, although the address may also be assigned to other lines as a nonprimary address. Address configurations recognized by the Telephony SPI include: