BT's directory inquiry services, 192 and 153, have stopped giving out telephone numbers after 46 years.
BT's 192 service is being replaced by more than 20 alternatives
Callers now hear a recorded message with a freephone number that selects at random the number of one of the more than 20 replacement services, which all begin with 118.
Telecoms watchdog Oftel, which is behind the change, says it will mean improved value for the £300m callers spend on 700 million calls to the service every year.
But while one of BT's competitors is offering nearly a million registered customers free directory inquiry calls, a leading mobile phone company is charging up to £2.50 a minute.
Consumers' Association senior researcher Jenni Conti is urging Oftel to inform callers about the different tariffs.
"Consumers are likely to be bamboozled by choice," she said.
New companies have spent millions of pounds on marketing campaigns.
Reports suggest Britain's directory enquiries market, the largest in Europe, could be worth up to £1.3bn within three years.
Spicing it up
And as companies jostle for their share of the pie, research suggests BT's own 118 offering is falling lower on callers' radar screen.
Among the major spenders in the 118 branding war have been The Number, Conduit and BT.
The Number, owned by the US call centre group InfoNXX, has very noisily sought to create a distinctive brand, with its two moustached 118 runners campaign on television, billboards, ice-cream vans throughout the UK and a T-shirt campaign through the Cancer Research charity.
"Number memorability is the key thing," The Number's marketing director Alex Harris told BBC News Online.
Ms Harris said the comic campaign is making a potentially uninteresting product a saleable commodity.
The Number's runners are looking for quirky cult status
"It's a fairly dull industry, which we wanted to spice up with an engaging, fun vehicle."
The Number has a marketing budget of £20m for the year, almost half of which it is spending in August and September.
The majority is being spent on a large television campaign involving the runners - and the supposedly 'old hat' 192 service.
"Poor old 192 has taken its pipe and slippers and is off to retirement," said Ms Harris.
BT is responding by spending its £10m marketing budget on reinforcing a 'tried and tested' brand.
Quality versus price
"We're not just a number. We're BT," Simon Lubin, BT Directories' head of marketing told BBC News Online.
"It's one thing spending £20m getting people to use your number, but if the service is bad, they won't use it again," he added.
Mr Lubin conceded, however, that the onslaught of 118 competitors has already forced the group to reduce staff numbers in its call centres.
"Inevitably we're going to lose market share... and we have adjusted the capacity at our call centres," he said.
BT phone home
The Number's aggressive marketing campaign appears to be working. Research group Millward & Brown has suggested consumer awareness of the different numbers from March to July has shifted considerably in the runners' favour.
"The spend is becoming more similar... but by July 118 118 had significantly more awareness," said a spokeswoman for the research group.
The Number has concentrated its efforts on the run-up to 24 August while BT is focusing on customers after the switch.
"The world won't end on Sunday," said Mr Lubin.
Conduit is one of the few to compete on price
"People won't change the habits of a lifetime. We need to keep reminding people."
Research from consultants Deloitte & Touche also suggests the attack on BT's pricing policy is unfounded by a number of competitors.
"We looked at the typical length of a call for 100,000 calls" Deloitte & Touche told BBC News Online.
"BT is firmly in the middle of the pricing for an average length call below 68 seconds."
BT also offers a free directory enquiries service on its website.