British Telecom is preparing for 3 million to 4 million people an hour at peak times to dial a wrong number after tomorrow's change to the telephone number system - the biggest shake-up for 25 years.
Despite more than two years' warning, a £16m television and press advertising campaign and eight months of parallel running, BT fears misdialling could be as high as 50 per cent.
The holiday weekend was chosen for the change as telephone traffic is half that of a normal weekday. Between 30 million and 40 million national number calls are made each day. The crunch comes on Tuesday morning when industry and commerce resume after the Easter break. Each misdialling caller will be played a recorded message from tomorrow, or Phoneday as BT calls it.
A number 1 in front of all national codes, so 071 becomes 0171, will give the expanding telecommunication industry another 10 billion lines. An acute line shortage in Leeds, Sheffield, Nottingham, Leicester and Bristol has meant those cities will have all-new codes. The international access code will change, too, from 010 to 00, bringing Britain into line with other European Union countries. BT said market research had shown more than 90 per cent of individuals and companies were aware of the changes but that a quarter of businesses had yet to alter telephone equipment or computer records. "More than one-third of all national calls are already using new codes," Alan Croft, project manager for the switch, said.
"But we are still concerned about a minority of businesses which appear to be waiting until the very last minute."
All voice connections will receive the message, but facsimile machines and computer modems cannot recognise voice sounds and most will keep dialling automatically. Part of the reason for expanding the numbers available has been the rapid growth in technology, including the 3.8 million mobile phones. However, mobile phone numbers, freephone and information and entertainment lines, will not be changing.
Oftel, the telecommunications regulator, has started to divide the billions of new lines that will become available with the allocation of 02-09 prefix codes. It has decided the 04 code will go to new mobile phones, 08 to special tariff lines and 07 to a new system of "personal" numbers, for individuals to take with them if they move.
Public consultation is about to begin on the development of a geographic system that will eventually take the 02 code and absorb demand for extra phones.
BT's checklist for customers advises ensuring all security alarms have been adjusted, checking numbers in phone and fax memories, changing any code diversion numbers and updating records and stationery.
Businesses praised BT's campaign to make people aware of the changes and reported a low level of complaints compared with the introduction of the 071/081 London codes. Lynne Imeson, director of policy at Nottingham Chamber of Commerce - code changing from 0602 to 01159 - said most firms had responded well.
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