Revenue and Customs admits more needs to be done to improve performance
HM Revenue and Customs failed to answer about 44 million phone calls last year, Whitehall's spending watchdog says.
The National Audit Office called the performance of 31 customer "contact centres" during 2008/09 "unacceptable".
Despite employing the equivalent of 10,500 full-time staff at a cost of £233m, it still failed to pick up 43% of the 103 million calls received.
HMRC said its performance had improved in 2009-10 and that it was "committed" to providing a better, cheaper service.
During the busiest periods of the year - such as the tax credit renewals peak in July - just one in three calls was actually answered, the National Audit Office (NAO) said.
'Evidence of chaos'
Callers who did get through had to wait an average two minutes for a reply - or almost four minutes if they were ringing at peak times.
By contrast, the best practice target in the private sector is for 90% of all calls to be picked up within 10 seconds.
Although the latest figures for the first half of 2009/10 have shown some improvement by HMRC, the NAO said that more that 27% of calls were still not getting a reply.
For the Conservatives, shadow chief Treasury secretary Philip Hammond said these failures would affect some of the most vulnerable people in the country.
He added: "This is more evidence of the chaos at HMRC. Every missed call to the Revenue represents someone who has been let down by Gordon Brown's bureaucracy."
An HMRC spokesman said that, while its performance had "significantly improved" in the first half of 2009-10, more needed to be done.
He added: "That's why we've committed to answering 90% of our calls, the industry standard, at 30% less cost by March 2012."