HM Revenue and Customs is setting £45 million aside to improve customer service after figures showed that 7.2 million phone calls made to it in the last year went through to a "busy" tone instead of the caller being able to speak to someone.
Between April 2014 and March 2015, 64.7 million calls were made to HMRC - of which 46.9 million were successfully answered. This means that a total of 72.5% calls were answered across these months, falling short of the revenue body's target of 80%.
More than one in 10 (11%) call attempts made last year went straight to a busy tone, with around 7.2 million call attempts ending in this way, the figures showed.
Some months were particularly stretched, with as many as one in five (20.8%) customers hearing a busy tone and being unable to join a phone queue in September 2014.
The £45 million allocation will come from the current spending settlement and is not additional revenue from the Treasury, HMRC said.
The money is paying for around 3,000 additional staff to join customer service teams, on top of around 2,000 staff who are being moved over temporarily from other parts of HMRC to help with the tax credits deadline and letters and forms.
HMRC said that calls tend to peak around key deadlines, such as January 31 for self-assessment and July 31 for tax credit renewals.
Lin Homer, HMRC chief executive, said: "Despite our best efforts, our call performance hasn't been up to scratch and we apologise to all those customers who have struggled to get through to us.
"Good customer service is an absolute priority for HMRC. We set ourselves the target to answer 80% of calls, to provide a more consistent level of service across the year and to reduce peaks and troughs in service levels between busy and quieter times."
Ms Homer continued : "We have also invested in new telephone equipment, which lets us switch calls to many more offices, not just take them in contact centres, so more of our staff can help customers at the busiest times.
"Our new online services are also giving customers new and better ways to deal with HMRC and I urge all customers who can go online to do so. For services like tax credits, it's quick, simple and can be done anywhere any time, including from a smart phone."
So far this year, 265,000 tax credits customers have already renewed online, compared with 97,000 at the same point last year.