Many users choose to dial people, fax machines, bulletin boards, and other entities by selecting their names from an address book. The actual number that is dialed depends on the geographic location of the user and the way the line device to be used is connected. For example, a computer may have access to two lines, one connected to a PBX, the other to the telephone company's central office. When making a call to the same party, different numbers may have to be used. (To dial through the PBX, for example, the PC may need to dial '9' to get an outside line, or a different prefix may be needed for a call made through the central office.) Or, a user may make calls from a portable computer and want to use a single, static address book even when calling from different locations or telephony environments. The TAPI address translation capabilities let the user inform the computer of the current location and the desired line device for the call. TAPI then handles any dialing differences, requiring no changes to the user's address book.
A related topic is the handling of international call progress monitoring, which is the process of listening for audible tones such as dial tone, special information tones, busy signals, and ringback tones to determine the state of a call (its progress through the network). Because the cadences and frequencies of call-progress tones vary from country to country, the service provider must know what call progress to follow when making an international outbound call. Therefore, applications specify the destination country code when placing outgoing calls.
To deal with directory numbers, TAPI defines two common formats. One is the canonical address format, the other is the dialable number format. The canonical address format is intended to be a universally constant directory number; it is the directory number one would like to store in the address book, and never change. TAPI contains operations (address translation) to take addresses in the canonical address format and convert them to the dialable number format. A side effect of this conversion process is to extract a country code identifying the target country for the call. All addresses that appear at TSPI have been translated to dialable address format (see Dialable Addresses). Functions that involve dialing include the extracted country code. This country code can be used by the service provider to provide call progress determination that is independent of national boundaries.