At the end of November, BT introduced a new service called Calling Line Indentification (CLI), which can transmit your phone number unless you take steps to prevent this happening.

There are two parts to CLI Call Return and Caller ID. Call Return can be used by any BT user whose phone is on a digital exchange and is a free service. It works like this: the number of the last person to call you is stored, even if they didn’t manage to get through. This could be useful if someone calls while you’re in the shower, or the caller rings off before you get to the phone. You simply pick up the handset, dial 1471, and a recorded voice tells you the number of the last caller.

Call Return will also reveal ex-directory or confidential telephone numbers, unless a person dials 141 before the number they are calling. Teachers and other educational workers who occasionally call parents or students from home, should be aware that their private number will be revealed unless they dial 141.

If you want a permanent block on your telephone number being sent, you can call your local BT customer services branch (150 for residential numbers, 152 for business) to arrange this. If you do ask for a permanent block, be sure to ask for what is known as a 1470 option. Dialling this will then enable your number to be transmitted during a call.

With Caller ID you can see the phone number of the person calling before you pick up the handset. To use it, you need to pay a subscription of Pounds 4 per quarter, and have a special phone (around Pounds 90 to buy or Pounds 9 per quarter to rent) or add-on box which plugs into your existing phone line (Pounds 50 to buy or Pounds 6 per quarter to rent). Both have an LCD screen which displays the number, and the phone can also store details of around 30 phone calls.

Caller ID would be useful for screening calls, and schools could check phone messages. A student ringing in to say he was at home ill in bed while skiving at a friend’s house, would soon be found out. Another option would be to use Caller ID to screen the calls of parents who persistently ring up and complain. Not that any school would wish to use the service for this purpose, of course.