The introduction of the services beginning with 118 has also led to a drop in the number of calls to directory enquiries, the report says. More than a third of callers say they now use directory enquiries less, although the study found this may be due to switching to alternative sources such as the internet.
Despite its criticism, the NAO felt Oftel handled the actual liberalism process well. Sir John Bourn, head of the NAO, said: 'The outcome of this market liberalisation for the residential caller has been more choice and innovative services but also an increase in average prices and uncertainty over improved quality.'
A mystery shopping exercise carried out by Ofcom in November last year found 83% of calls to BT's 118500 resulted in the correct number, compared with an average of 86%. The Number's 118118 was 94% accurate but is more expensive than BT, based on a separate survey by the NAO.
Matthew Dearden, general manager of directory enquiries at BT, said: 'BT's 118 500 service has great accuracy and very high customer satisfaction. BT therefore disputes Ofcom's mystery shopper accuracy results.
'The good news is that throughout the confusion, BT has continued to provide a service which remains as reliable as the 192 service we provided for 47 years.
'We routinely sample our services to check performance levels and our internal data shows the performance of 118 500 today is as good as 192 two years ago.'