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With the possible exception of customer originated trace - which is a powerful deterrent and detection system for malicious calls - these services are intended to generate fresh revenue streams. Apart from CND (Caller ID), which requires special customer equipment, all the functionality resides in the software of the telephone exchange. While there are some management costs associated with these services, the primary cost is the capital expense of purchasing the appropriate software from the exchange manufacturer. Consequently, revenue from these services is largely profit for the carrier.
Call waiting is a widely appreciated facility, enabling a conversation to continue whilst still being able to receive an incoming call. Some carriers decided to provide it as a free service, rather than as a paid service. This was probably based on the extra revenue which was generated by the calls which were received in a call waiting scenario being more profitable than the revenue raised by its more limited adoption as a paid service.
CND (Caller ID) is a service that is notable for the controversy it generated. Carriers insisted on its introduction on an opt-out basis: that the caller's number would be presented to CND customers they called unless the caller took specific action per call, or to have their service configured to maintain their pre-existing level of privacy. In California, a state in which the awareness of the privacy problems of such disclosure is relatively high, by mid 1998 approximately half of all customers have chosen not to allow their numbers to be sent by default.
Not so well known are the problems that Calling Number Display causes for corporate telephone customers using PABXs. For instance their outgoing calls may be made on lines with numbers which bear no relation to their directory number in the telephone book, nor to the particular staff member making the call, and so confusion is caused when calling CND customers. Calls returned to those numbers will typically not reach the person who made the original call, and will often go to lines that cannot receive incoming calls.
PABXS and Key Systems
A PABX (Private Automatic Branch Exchange) is an important piece of infrastructure for almost any large organization (see chapter 6, on page 5, on how the similar services can be provided with Centrex). A PABX has a certain number of lines to the carrier's exchange, and a larger number of internal lines to phones on the desks of the organization¿s staff. Calls within the PABX have nothing to do with the carrier and so incur no costs. Incoming calls, typically on one group of exchange lines, are identified by the exchange with an extension number, so that the PABX can route them directly to the correct internal telephone. Outgoing calls are typically made on another set of lines, and the PABX may automatically alter the number dialed by the user, for instance to add suitable prefixes to long distance and international numbers so that the calls are placed with the most cost-effective long-distance carrier.
Modern PABXs usually connect to the network via one or more PR-ISDN lines. Each such line (which may be delivered by a fiber pair or two copper pairs) carries 30 telephone calls (23 in North America) as well as a digital signaling channel to the exchange. This is by far the most efficient and flexible method of connecting to the network. The ISDN protocols include the transport from the exchange to the PABX of the caller's number (if its CND/Caller ID display was authorized), and they have the potential to affect the number displayed for outgoing calls made via the PABX.
Analogue PABXs are still widely used. Their connection to the network is typically via multiple twisted pair lines. The copper pairs are the same as those used for ordinary analogue circuits, but the signaling arrangements for incoming and outgoing calls are rather different. The carrier exchange must be able to signal to the PABX which extension the call is meant to go to, for instance based on the last two digits of the dialed number which falls in the 100 digit range of that organization¿s telephone numbers. The most common approach is also the least efficient: to use decadic pulse dialing. This takes two or three seconds and is sometimes audible to the caller as a series of muted clicks. The faster but less frequently used alternatives both use special tones to convey the extension number. Analogue PABXs cannot receive Calling Number Display information from the carrier exchange.
The term 'Key System' refers to small business telephone systems which do not have the sophisticated functions of PABXs and which connect to the network via ordinary POTS analogue lines. These may have multiple extension phones, and enable calls between those phones, but their interface to the carrier exchange is simply by multiple analogue POTS telephone lines.
Payphones
Payphones may be owned and operated by carriers, or by separate companies. They may accept payment with coins, magnetic cards, electronic 'smart' cards (cards with an inbuilt integrated circuit) or by the use of a credit card.
In the case of credit cards, the billing is carried out by the carrier's network and computers, but for coins and smartcards, the phone must be told by the network of how the call is being charged for as it progresses. Since payphones connect via an ordinary twisted pair line, similar to a POTS phone, there is no alternative channel of communication, such as ISDN's D channel, on which such information can be transferred.
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