The government is continuing to extensively use higher rate telephone numbers for customer contact lines, despite efforts by departments to reduce their use, according to a report from National Audit Office (NAO).

The NAO said departments had "inconsistent approaches" towards the replacement of 0845 numbers with lower cost 03 alternatives.

The Department of Health is the only major department to rule out the use of numbers costing more than the geographic rate, although these remain in use for some GPs.

In 2012-13 the Department for Work and Pensions (DWP), the Department for Business, Innovation and Skills and HM Revenue and Customs had the highest proportion of higher rate numbers.

The NAO found callers to higher rate lines paid £56 million in call charges in 2012-13. Callers spent a total of 880 million minutes on calls, of which 402 million minutes were spent waiting to speak to an adviser at a cost of £26 million.

The NAO estimates the value of callers' waiting time to be £100 million.

The NAO said departments also do not offer improved monitoring of services to callers in return for higher charges. There are 99 lines for which departments have a target for the proportion of calls answered. Of these 78 did not specify a time by which most calls should be answered.

"Long waiting times and dropped calls increase the cost of calls and exacerbate the burdens on vulnerable citizens," said the NAO. "Although 86 percent of higher rate lines serving vulnerable groups offer some form of cost mitigation such as call back, there is limited sign-posting of alternatives by government," it said.

Some departments, however, says the report, have taken "substantial steps" to reduce the burden on callers. Since April 2013, HM Revenue & Customs has introduced 03 numbers for its busiest higher rate telephone lines and will phase out higher rate numbers "over time".

The DWP has negotiated free calls from mobiles to its benefits claim lines on 0800 numbers, which "is a considerable saving to callers".

The NAO found though that departments do not monitor the revenues that third party higher rate phone providers receive.

The NAO estimates that changing all of government's higher rate numbers to 03 numbers would save callers £29 million a year, and cost government £7 million a year "including loss of implied revenue share". Changing all numbers to 0800 numbers would currently save callers £46 million a year and cost government £21 million a year.

Amyas Morse, head of the National Audit Office, said, "Callers do not receive a better service from higher rate numbers and many callers are put off calling government phone numbers altogether.

"The most vulnerable callers, such as low-income households, face some of the highest charges. Each department needs to take a clear approach to using higher rate numbers and protecting vulnerable callers, and improve their understanding of how to get the best value from telephone services for both callers and taxpayers."