The study found that the best times to call HMRC is between 8.30am and 9.30am or 12pm and 12.30pm where call waiting times averaged four and half minutes.
The worst times to call are before and after working hours, with taxpayers put on hold for an average of 12 minutes before speaking with and expert at the Revenue between 4.30pm and 5pm.
PfP added that some taxpayers calling during this time waited on hold for close to 25 minutes prior to being connected to a HMRC adviser.
Kevin Igoe, managing director at PfP said, “Taxpayers understandably want to contact HMRC when it is most convenient for them but this is likely to be at those peak times when call waiting times can reach over 20 minutes.”
PfP added that despite some improvements to call waiting times, there are still concerns that HMRC are unable to cope with the volume of calls they receive at peak times or as tax deadlines approach.
PfP also highlighted concerns that long call waiting times could lead to more mistakes being made on tax returns as taxpayers choose to hang up rather than wait on the line for HMRC.
“There is a real concern that taxpayers will make mistakes on their tax return, if they can’t get hold of HMRC- which could lead to serious consequences,” Igoe warned.
“Tax returns can be complex for individuals so it’s important they have access to advice and guidance from a professional.”
“For those taxpayers already in disputes with HMRC, difficulties contacting the Revenue will be an additional worry.”
“As HMRC continue to crack down on the underpayment of tax, they in addition need to ensure they provide sufficient quality customer service to deal with the subsequent queries from taxpayers,” Igoe added.
The research follows repeated criticism of HMRC’s service standards over the last 12 months after call-waiting times increased dramatically last year following a raft of staff cuts.
The report revealed that the Revenue's helplines cost customers £1 for every £4 saved, with people waiting for their calls to be answered for 47 minutes on average.
In addition more than a quarter of people gave up on their calls due to the “unacceptable” waiting times, the MPs said.
The PAC said HMRC must ensure it can maintain service standards before making further cuts to staffing after service levels collapsed in 2014/15 and early 2015/16 when the tax authority sacked 5,600 customer service staff in a bid to cut costs.
HMRC was forced to recruit 2,400 new staff at the end of 2015 to recover service standards, after average-waiting times tripled compared with previous levels, resulting in taxpayers incurring direct call charges while on hold for long periods of times.
HMRC has since vowed to shorten call waiting times and improve customer service.
A spokesperson for the Revenue added that the data used for the study was for a period when improvements were just beginning to set in and that the results are now outdated.
The spokesperson added, "We've always been very clear that there are times of the day which are quieter than others but wait times have improved across the board due to extra staffing and new online services that reduce the need to call. We move extra staff onto the phones to meet our customer demand during our peak periods. This means we don’t pay for extra staffing all year round but we are there for people when they most need us.
“Customers get through to us in less than five minutes now and they can also ask general questions on twitter, or go online to view their tax details from a smartphone anywhere, anytime, or speak to our customer service team via webchat. It’s never been easier to get help with your tax affairs.”
Caroline Miskin, technical manager in the ICAEW tax faculty commented, "HMRC has already acknowledged that its performance in 2014/15 and the first part of 2015/16 was not good enough and this was evident from ICAEW research of members in 2015. HMRC‘s telephone performance did improve significantly towards the end of 2015/16 and we understand that this improved performance has continued in the first part of the current tax year. HMRC’s target has been increased to 85% of calls answered within six minutes in 2016/17 – six minutes is an average and individual taxpayers may still experience significantly longer waiting times. A £71m investment in HMRC customer service was announced in Budget 2016 and a number of initiatives including extended opening hours are being considered."
Miskin added, "The improvement since the end of 2015 is welcome but needs to be maintained, with a clear target for what is an acceptable level of customer service and how this will be achieved. It is not clear how HMRC is going to maintain and improve customer service given the cuts in funding and staff expected by 2020/21. HMRC appears to be relying on a dramatic reduction in the number of phone calls to its helplines, but at the moment the jury is out on how personal taxpayers will respond to digital services and the impact that it will have on reducing the number of calls. There needs to be a robust improvement plan in place that is flexible enough to adapt as circumstances change."