The boss of HM Revenue and Customs (HMRC) has defended its poor record on answering telephone calls.
It follows a report from Citizens Advice, which said thousands of people were waiting 47 minutes to get their calls answered.
"I don't think the overall trend is worsening," Lin Homer, HMRC's chief executive, told MPs.
The National Audit Office (NAO) also said recently that too few calls were being answered within five minutes.
"We aren't answering enough calls within five minutes, but we are now doing more 'once and done' calls," she said, meaning that taxpayers only needed to phone once.
"We have already apologised for what we see as a failure in our performance at the end of last year, and the first couple of months of this year," she told members of the Public Accounts Committee.
In June this year, she apologised for the service, which she admitted was "not up to scratch", after figures showed that a quarter of calls went unanswered.
As a result of "once and done" calls, she said that some conversations were getting longer. And she said HMRC was "aiming to get better".
The total number of calls answered by HMRC fell from 79% in 2013/14, to 72.5% in 2014/15, according to the NAO.
And it said that just 39% of calls were being answered within five minutes.
'Paying the price'
The Citizens Advice study showed that over the last year, 11,500 frustrated callers turned to Twitter to complain.
On average, those who tweeted had to wait an average of 47 minutes before their call was answered.
HMRC said the survey was "unscientific" and "out of date".
In one case, someone tweeted that they had tried to get through to HMRC on four occasions and waited an hour each time.
Citizens Advice calculated that hanging on the line to HMRC for 47 minutes would, in many cases, cost consumers £4.66 in call charges.
"People are paying the price for not getting through to HMRC," said Gillian Guy, the chief executive of Citizens Advice.
"From fines for not completing a tax return in time, to under or overpayments for tax credits, people can be left out of pocket because they cannot speak to HMRC on the phone."
However, HMRC said it had recently taken on an extra 3,000 staff to try and lessen the delays.
"We are sorry that some customers have struggled to get hold of us, but this unscientific and out-of-date survey of tweets does not represent the real picture now," said an HMRC spokesperson.
"In reality, answer rates on our phones are improving and wait times are falling."
Lin Homer said new telephone systems had also been installed, which would help reduce waiting times.
The Citizens Advice study looked at 34,000 tweets made between August 2014 and August 2015.