More than 10 million numbers are set to change tomorrow in the third major shake-up in less than a decade.
The move will see numbers switch in Cardiff, Coventry, inner and outer London, Portsmouth, Southampton and Northern Ireland.
Just four years after PhoneDay, which saw the introduction of a "1" into national area codes, a far more complicated change is in store with local numbers altering as well.
The changes are so complex that telephone companies have clubbed together to form an organisation called the Big Number to help people understand them.
Businesses are being targeted first to prepare for the new numbers, then a major awareness campaign will be launched for the public in the autumn.
Current numbers will still work until next autumn to give people time to get used to the new system.
The move is set to anger business people, who have already been hit by the costs of preparing for the euro currency and the millennium.
A recent Big Number poll found only one in four companies was ready for the changes.
But Oftel, which instigated the move, said it was vital to keep up with the growing need for new telephone numbers.
In February, however, the Trade and Industry select committee criticised the telephone watchdog for failing to consider alternatives to another upheaval, and for not consulting customers.
The seven zones changing will each have new three-digit area codes starting "02". For example inner and outer London - currently 0171 and 0181 - will become 020 and Portsmouth and Southampton - currently 01705 and 01703 - will be 023.
At the same time, the five, six or seven digit local numbers used until now will be changed to eight digits by adding extra figures at the beginning.
On local calls, the new eight-digit numbers will not work until 22 April 2000, unless used with the new national area code.
Oftel is also creating millions of new potential numbers for future use, with the area codes 03 to 06 being left blank until needed. Changes are also planned for mobiles, pagers and freephone numbers.