Computerlink: Eyeing a telephonic peephole - Soon you will be able to tell who is ringing before you pick up the phone. Steve Homer looks at the pros and cons

Our relationship with the telephone is about to undergo a fundamental change. Next year British Telecom plans to offer a new service - Calling Line Identification (CLI) - which will let you know who is calling before you pick up the phone. The reverse will also be true: even if you don't subscribe to the service, people who do will be able to know your number when you are calling them.

This Wednesday is the last chance for members of the public to address comments on this innovation to Oftel, the telecommunications regulator.

CLI, when we have got used to it, may offer tremendous advantages. But it will remove a layer of privacy between the caller and the called that has existed since the telephone become part of our everyday lives.

CLI will act like a peephole in the front door, allowing you to see who is calling and then decide whether you want to talk to them. The system will radically reduce the number of malicious calls, especially as BT also proposes a cheaper CLI service called Call Return. With Call Return, if you miss a call, you can dial a special number and the exchange will tell you the number that has just called. This will also work even if you answered the call.

Subscribers with the full CLI service and the right phone will be able to 'trap' several numbers when they are out. In the United States, where CLI has been available for several years, some phones can trap 50 numbers or more.

Phones will also be able to give special rings for different callers. A particular ring could be allocated to friends of teenage offspring, sparing parents continual running to the phone when the children are out; or special rings could be allocated to family and friends. In some US states diversion of specific calls to special numbers is possible.

But it is the use of the service outside the home that is more controversial. If you were to call a commercial company that uses the service, it would be able to capture your number. This could be used by unscrupulous companies to pester susceptible individuals, such as the elderly. For these reasons, the service will be introduced with 'call blocking'. It will be possible to prevent your number ever being given out - 'line blocking' - but you will need to ask for this specifically. Or, by dialling 141 before you make a call, your number will not be sent out just for that call. To stop callers abusing this system, the person receiving the call will be able to refuse to answer a blocked call unless the calling party reveals their number.

While it is easy to see disadvantages to CLI, there are many advantages. When you call a company such as British Gas, your details could appear on screen automatically when you ring from home or a regular phone, speeding up the service. It will also facilitate the company calling back when your query cannot be answered immediately.

For the emergency services, CLI offers the possibility of responding more quickly to calls, and it will cut down the number of hoaxes. However, for confidential helplines such as the Samaritans, Childline and the like, and police information lines, CLI poses a problem. The organisations can, of course, choose not to install CLI equipment, but how will callers know this? The solution may be for the telephone company to guarantee that CLI information is not forwarded on certain lines.

In the United States, CLI has been introduced in all but six states. In Pennsylvania the service has been outlawed under state privacy legislation. In some states, the local phone company has not been able to introduce the service for technical reasons. But according to Suzanne Hutchings, an attorney at the Federal Communications Commission, California's telephone company, Pacific Bell, has not introduced the service because the state government wanted a high level of 'subscriber understanding' before the service could be launched. Ms Hutchings says Pacific Bell felt that the strictness of these requirements made it impossible to introduce the service economically.

'The education issue is very important. It is the responsibility of the provider and the regulator to get it right,' says Vivienne Peters, chief executive of the UK's Telecommunications Users Association. Mrs Peters believes that when BT introduces the service all residential lines should be blocked to CLI until users ask for their numbers to be released. This would guard against uninformed customers releasing their numbers unintentionally. Mrs Peters thinks there are real benefits from CLI, particularly in dealing with companies, but that to prevent abuse of the service the Data Protection Act may need to be tightened.

The office of the UK's electronic data watchdog, the Data Protection Registrar, agreed, favouring introducing CLI with 'call blocking (as) the default', but Council of Europe recommendations and a European Union draft directive now 'make this impractical', according to the registrar's 1992 report. However Francis Aldhouse, the deputy data protection registrar, believes there may be some leeway and Britain may able to go its own way and introduce the service with initial line blocking.

Mr Aldhouse is determined that companies should not be allowed to misuse the system: 'If it is not obvious what a company wants information for and what it is going to do with it, the caller should be told.' He believes there would be no chance to sell on consumers' numbers legally without callers being informed, under current data protection legislation.

At present Oftel opposes introducing the service with initial line blocking and backs BT's plan to introduce the service with everyone automatically sending out their number. Oftel believes that it will be impossible to establish a market if no one is sending out their number in the early days. But according to Fod Barnes, consumer policy adviser to the director-general of Oftel, the regulator will impose onerous education requirements on the telephone companies and believes the public has little to fear. 'Most calls we make are to people who already have our phone number and from trials BT has carried out and experience in the United States, people want to use number blocking very rarely.'

BT, in its submission to Oftel, appears to be aware of the problems it faces in introducing the system. It declares itself committed to free line and call blocking and an extensive education campaign. But, overall, the company feels that CLI will be a useful, and profitable, service. 'More than 80 per cent of the customers in our trial felt the service was so useful we should introduce it nationally as soon as possible,' says Carol Rue, manager of the Network Services Development programme. 'But we realise this technology will change the relationship between callers and the people they are calling so we are absolutely committed to making sure our customers understand the changes that are taking place.'

Comments about CLI should be made to: Fod Barnes, Oftel, 50 Ludgate Hill, London EC4M 7JJ, by 15 December.