Dial M for mix-up as BT's big changeover meets the unknown bug in switchboards

The woman's voice is calm and understanding, unfailinglypatient as she explains your error. "London codes and numbers have now changed," she coos. "For inner London please redial, adding the number seven in front of the old seven- figure number ..."

It is just as well that the woman's voice sounds so serene. Over the past three days, millions of people within the capital have tried to make a phone call only to hear her recorded message, politely telling them to try again.

At one point on Saturday morning, several hours after the latest batch of new telephone codes came into operation, one in three calls within London was being misdialled. One can only hope the anonymous woman gets paid every time her voice is heard.

But things are likely to get worse before they get better. With most people returning to work today after the Easter weekend, the phone companies are preparing for further chaos. A spokeswoman for the Big Number, the firm set up to handle the code switch by the 40 or so telecommunications companies affected by the changes, said: "We cannot predict what is going to happen but it is something that we are concerned about."

But the problem of having to deal with Britain's most complicated number switch has been made worse by an only recently discovered glitch in the switchboards of thousands of businesses. Given the snappy title of "the switchboard bug" by the Big Number, the problem means that certain switchboards that have not been programmed to handle the new numbers are likely to have difficulties when people make local calls.

Over the past few days, many people calling a central London number from another central London number by adding a seven to the old seven digit number have been told to redial, this time prefixing the old seven digit number with the number seven - which is what they have just done.

Michael Dixon, chairman of the Telecoms Managers Association, a professional body for systems managers, said: "Switched-on businesses, especially those with telecoms managers, should have eliminated the switchboard bug before or during Easter.

"The trouble is, not every business has a telecoms manager and if they haven't reprogrammed their equipment ... they could face baffled staff on Tuesday morning."

The latest spate of phone misery began in January 1997 when the Office of Telecommunications (Oftel) announced the need for millions of extra phone numbers. The pressure came from the growing demand for business numbers, mobile phones, faxes and second phone lines to use to connect to the internet.

Two years earlier, in 1995, when London numbers were given an extra '1', Oftel said there would be no need for further changes in the foreseeable future. It had rapidly become clear this was not the case.

And the pressure is not abating. While London now has 20 million numbers available, Oftel says that, with the current growth in demand, it will only be a matter of a few years before the introduction of the first London code not beginning with a 0208 or 0207.

David Edmonds, the director general of Oftel, said last week: "When we look back on this in three to five years, we will see Britain with the most rational and logical numbering system in the world."

The winners in all of this are supposed to be businesses and the individual customers who are demanding the extra numbers. But there is also a cost to all of this. Business leaders say there is concern that companies risk losing trade if customers have problems getting through to them.

Oftel itself admitted a week before the code change that millions of people were unaware of the new numbers, despite a £20m advertising campaign by the Big Number. British Telecom staff rang 11.5m customers to inform them of the changes.

Andrew Parkinson, a spokesman for the British Chambers of Commerce, which represents 126,000 businesses across Britain, said: "I think that there is a problem because firms do not know what is going to happen. We were told five years ago that there was going to be no need for another change and yet we are having to deal with this.

"The fact that nobody really knows what is going to happen is frustrating - that and the potential loss of customers."

There is also another, more obvious, cost to business. Estimates suggest that individual small businesses have had to pay between £500 and £2,000 each to replace stationery, change facia boards and repaint signs with the new numbers.

Helen Conway, the chief executive of the Chamber of Commerce in Cardiff, one of the five areas outside of London that have also received new codes, said: "I think that business would be very angry if there was another change.

"They cannot guarantee how long this new number will last. It just seems to be a mess. When I rang directory inquiries last week I was still being told the old number.

"They said they would be giving out the new one until 1am on Saturday but that doesn't seem to make any sense."

In Wales there was additional concern that Oftel did not offer business and individual customers the option - mentioned in the consultation paper - of having a separate code for the entire principality. Mrs Conway said: "This was supposed to be the option that would give us the most numbers, so I don't know why we did not get that."

And this is not the end of it. By 2005 it is expected that Aberdeen, Bournemouth, Bradford, Brighton, Cambridge, Derby, Guildford, Middlesbrough, Oxford, Preston, Stoke-on-Trent, Wigan and Wolverhampton, will all be given new codes.

Oftel said last week that it was happy with the publicity campaign and that it was "peaking at the right moment". Up to last Thursday one per cent of all the calls it was receiving was in relation to the change. That may change today as millions return to work to and decide to vent their frustration with a call to the regulator - unless they misdial and encounter that familiar serene voice.