HM Revenue and Customs failed to answer about 44 million calls last year, the Whitehall spending watchdog has disclosed.
The National Audit Office (NAO) described the performance of HMRC's 31 customer "contact centres" during the course of 2008-09 as "unacceptable".
Despite employing the equivalent of 10,500 full-time staff at the centres at a cost of £233 million, HMRC still failed to pick up 43% of the 103 million calls it received during the year.
During the busiest periods - such as the tax credit renewals peak in July - just one in three calls (33%) was actually answered.
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.
That contrasted with best practice in the private sector where the target 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 than one in four calls - 27% - were still not getting a reply.
In the private sector, the benchmark is that no more than 10% of calls should go unanswered.
