Tens of thousands of small and medium businesses face a trebling of their phone bills in September if they have not already moved off their old premium-priced non-geographic numbers.
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The numbers affected start with 0870, 0871, 0872, 0873 and 09.
Opal, the B2B division of the TalkTalk Group, said up to a third of UK businesses could face increases of up to 200%, because they have not responded to new Ofcom rules.
Opal said a business that took 50,000 peak minutes a month on its 0870 numbers and received a rebate of over £1,500 could in future have to pay in more than £1,200 a month.
Ofcom warned in July 2006 that it was changing its rules to provide consumers with greater transparency over the cost of using 08XX and 09 numbers.
Many SMEs used the numbers to earn extra income through call rebates, often to finance enhanced call services. The new rules came into force on 1 August.
Opal's research into its 100,000 customers found that up to a third that use the numbers were unable to respond to the changes due to time and manpower constraints.
They have now been migrated automatically to a new tariff from 1 August where they will be charged for all calls to their 0870 numbers.
"This really could be a black September for thousands of businesses when the first bills arrive," said Opal finance director Geoff Wilson.
"Some firms are still calculating the impact of changing their 0870 number to an alternative, working out where the number is currently published and trying to understand the cost of changing literature, websites and so on," he said.
The Telephone Helplines Association has published a summary of the changes.
How to avoid the price hike
It's not too late to save your bacon from the 0870 debacle, says Opal:
Keep your existing number range, but use the revenue from it to pay for enhanced call services and back-office functions.
Change to an alternative sequence that generates revenues, but communicate the number change internally and externally.
Opal has published a fuller guide to the options.