Tax bosses have promised the service offered by public helplines will be improved, after it was revealed that almost a third of calls are getting cut off.
Research by consumer group Which? found that, in a sample of 100 calls, only 71 were not cut off with an automated message saying the service was "very busy".
Those calls that did survive this initial cut waited an average of 18 minutes to speak to someone, with the longest waiting 41 minutes.
The system's voice recognition also made mistakes when directing queries to other departments, with more complex phrases being misunderstood.
For example, when asked "do I need to pay tax on premium bond winnings?" the system asked if the caller was inquiring about changing a name or about a VAT surcharge notice.
The research comes in the run-up to the self-assessment tax return deadline of 31 January.
HM Revenue and Customs admitted the service "isn't good enough" and that new technology is being brought in to improve responses.
Which? executive director Richard Lloyd said: "With large numbers of people soon to be seeking help with their self-assessment tax return, we want to see HMRC doing more to monitor and improve their call-waiting times."
A spokesman for HMRC said: "HMRC receives over 40 million calls a year but we know that some of our customers can struggle to get through on our helplines at very busy times. This isn't good enough, and we are working hard to improve the range of services we provide.
"This year we are introducing new technology to help us answer more calls quicker at busy times, and we are improving the digital services we offer so that more customers can find all they need online.
"There is more to do, and we are committed to improving the service we offer all of our customers at all times, to help them find advice and support when they need it."
