How to avoid dialling a wrong number next week

Tomorrow is Phoneday, the biggest change to telephone numbers for 25 years. Here is what you need to know

On Phoneday, all area codes - except those for the five cities listed right - will have an extra digit "1" added after the initial "0". For example, Birmingham's code will change from 021 to - 0121, the Cardiff code will become 01222 and the 071 code for inner London changes to 0171.

Five major cities will be given completely new codes, and will have an extra digit added to the front of all numbers, in order to meet the exceptionally high demand for telephone services in those areas.

On Phoneday, the international code for making a call from the United Kingdom will also change, from "010" to "00" as part of a European harmonisation programme.

Some codes, such as 0800 for freephone numbers, including British Telecom's Phoneday helpline on 0800 01 01 01, 0860 for mobile phones and 0891 information and entertainment services, will not be changing on Phoneday.

BT call charges are not affected by any of these code changes.