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 desktop computer can 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 computer may need to dial '9' to "get out," 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. TAPI's 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. An application uses the lineTranslateAddress function to convert an address from the canonical address format to the dialable address format (see Canonical Addresses).
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 outgoing call. Therefore, applications specify the destination country code when placing outgoing calls.