Who's speaking, please?

People rarely stop to consider how strange those words are. When someone knocks at your door, you can look them in the eye before deciding whether to let them in. When you receive a letter, you can look at the signature before deciding whether to read it. But when the phone rings, you have to start a conversation without knowing the identity of the person at the other end.

The burden is on you to ask who is speaking. And the caller is under no obligation to answer.

Two weeks from now, all this will change. From 5 November, BT is to offer a radical service to the 22 million or so subscribers connected to its digital exchanges: it will tell customers the number of the telephone that is calling them.

In the industry this technology is known as 'Calling Line Identification' or CLI. BT is marketing it as two different services. Call Return will allow customers to find out the source of an incoming call, whether or not it was answered, simply by keying in a short code. This will be free.

The other service will be more useful, but it will cost pounds 4 a quarter.

When an incoming call arrives, you will be able to see the caller's telephone number, displayed either on a special telephone itself or on a separate box, before you pick up the receiver; the apparatus will cost from pounds 50 upwards to buy or from pounds 6 a quarter to rent. The more expensive machines will also show the last 50 numbers from which incoming calls, whether answered or not, were received.

Call Display and Call Return will have profound effects on the way we use home telephones. At the moment, the only way for private individuals to screen incoming calls is to make a caller leave a message on an answering machine, and intercept if it is someone they want to speak to. From 5 November, the screening process will require no more than a glance at the display.

Missed calls will also be a thing of the past. No longer will it be necessary to ring round friends to discover which one called while you were in the bath.

Perhaps most important, the heavy breather will become a dying species.

Those who live in fear of receiving malicious calls will find that the appearance of a number on their display gives them greater confidence in dealing with such people who ring. Those who make them will know that they have a much lower chance of harassing others without detection.

But it is the commercial uses of the new services that are most exciting.

Small companies will see obvious attractions. Pizza parlours will no longer be at the mercy of prank requests for a dozen extra-large pineapple pizzas, delivered to someone else's address. Plumbers will be able to find out if callers who rang while they were out still need their burst pipes mended.

Larger companies that can afford the technology to link their telephone systems to their data bases will put calling number identification to more spectacular uses still. Computer systems will be able to save time by displaying customers' account details when they call.

Banks, brokers and insurance companies will be able to offer services over the phone with a lower risk of fraud. Department stores will be able to use 'reverse directories' to track down callers, and send them mailshots about products relating to the department that they rang. And manufacturers will be able automatically to forward incoming calls to the customer's closest retailer.

What makes the introduction of these services all the more startling is that, for once, Britain is not lagging behind the United States by a decade, and indeed will lead the world. Calling Line Identification was introduced in New Jersey in 1987, after a similar service had been offered on US toll-free numbers for some time. It is now available across the US, with the exception of Alaska and Hawaii. But the fragmented nature of the US telecoms industry has reduced CLI's usefulness.

In Britain, details for exchanging CLI between BT and its competitors have not yet been finalised, so incoming calls from Mercury, mobiles and lines operated by cable television companies will not at first produce numbers on the display. But BT's share of the local market is so high that 80 per cent of customers will find they can identify 80 per cent of their calls after 5 November. This will give the services the critical mass they need. By the end of this year, CLI may well cover all telephone companies' digital exchanges.

But several issues remain unresolved. In the United States, CLI has been controversial because the privacy that old-fashioned telephoning gave the caller had advantages as well as drawbacks. Battered wives in hiding could use the telephone to call their husbands. Would-be suicides and drug addicts could appeal for help anonymously from counselling services. Informers could give information to the police in secret. And those interested in, say, double-glazing, could make telephone inquiries without fear of being pursued to their graves by rapacious salesmen.

It is for these reasons, among others, that a number of groups in the United States have claimed that CLI breaches constitutional rights to privacy, either at the federal or the state level.

Aware of the risks, BT is already trying to prevent the US fiasco from recurring in Britain. The flyers it has already sent out promoting the new services contain promises from the Samaritans and other organisations not to trace incoming calls. And it is allowing subscribers to mask their identities not only call by call, but also permanently by making a single call to the exchange. (BT has been careful to point out to aspiring hoax and heavy breathing callers that its own engineers and the police will still be able to trace calls.) The company boasts that the 450,000 customers who have already used these services in Elgin, Perth, Edinburgh, Bristol and elsewhere are less concerned than Americans about the right to keep their identities secret. A survey in Perth showed that three in four respondents saw no reason for any callers to withhold their numbers when phoning others.

Whether telephone subscribers will change their tune when CLI goes national remains to be seen. The disadvantages of revealing one's number will become apparent only when companies begin to put the new technology to use. BT must hope to win over its residential customers before any controversy breaks.

(Photograph omitted)