Tax bosses have promised improvements to public helplines after a consumer watchdog found almost a third of calls were getting cut off.
HM Revenue and Customs accepted its service "isn't good enough" at peak times following the research by Which? in the run-up to the self-assessment tax return deadline on January 31.
In a sample of 100 calls to the tax return and general inquiries lines, only 71 were not cut off with an automated message explaining the service was "very busy", the organisation said.
And even in those cases callers were put on hold and waited an average 18 minutes to speak to someone - with a longest recorded delay of 41 minutes.
The investigation also found glitches with a voice recognition system designed to direct queries to the right department.
Which? said that while it dealt easily with simple phrases, more complex requests were not understood.
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.
Which? executive director Richard Lloyd said: "We've found people could face lengthy waits or even be cut off when trying to get assistance from HMRC's helplines.
"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."
HMRC said it was sorry for delays to calls but insisted they were a minority of the large volumes it received and new technology was being introduced to improve responses.
A spokesman 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."