Half of all telephone calls to HM Revenue and Customs go unanswered - meaning many people with questions about the complicated UK tax system are being left adrift.According to a scathing new report from Parliament's Public Accounts Committee, HMRC answered three-quarters of telephone calls during 2011/12, a level already criticised as 'abysmal'. But in the first half of 2015 the number of calls answered fell even lower - to 50 per cent.In fact, the report claims that in 2014/15 only 39 per cent of calls were answered within five minutes. 'We are concerned that customer service levels are so bad that they are having an adverse impact on the collection of tax revenues,' the government says in its report.A spokesperson for HMRC responds: 'We explained to the committee that we hadn't provided a consistent level of customer service in the first half of the year and we had recruited around 3,000 new staff to improve service levels. But these customer service issues did not affect our ability to collect tax.'But according to its annual report, HMRC has cut 11,000 'full-time equivalent' posts since 2010.Mark Serwotka, general secretary of the Public and Commercial Services Union, says HMRC is struggling under the strain of severe budget cuts.'It has been abundantly clear for years that the department has cut too many staff and that services are suffering. The department needs major investment backed by real political commitment to tackle evasion and avoidance as an alternative to more damaging spending cuts.'The HMRC spokesperson adds: 'We are disappointed that the Public Accounts Committee has overlooked HMRC's record results, which include collecting a record £517 billion in tax revenues and further reducing the UK's "tax gap" - the difference between what is due and what is collected - to ensure it remains one of the lowest in the world.'Lack of accountabilityHowever, the government report slams the tax collection agency's ability to close this gap, pointing out that HMRC's own measure of the tax gap excludes aggressive tax avoidance schemes that are technically legal but go against the spirit of the law.HMRC also came under heavy criticism for its lack of accountability - in terms of both tracking aggressive tax avoidance and the amount of tax foregone via legal tax reliefs. Regarding the latter, HMRC lists 400 existing tax reliefs, but the Office of Tax Simplification says there are in fact more than 1,400.Finally, the report also said HMRC was being lax on wealthy offshore tax avoiders, prosecuting only 11 since 2010. Since a former HSBC employee leaked the Swiss bank account details of 3,600 potential UK tax evaders, for example, only a single person from the list has been prosecuted.'The vast majority of UK individuals pay what is due from them in tax. Those who do not must in future know that they could face prosecution if they deliberately seek to evade paying what is due in tax,' the report says.The spokesperson for HMRC adds: 'Tackling tax evasion is a top priority for HMRC and last year alone we successfully prosecuted a record 1,200 cases [mostly onshore], resulting in 407 years of custodial sentences.'We also routinely publish the number of tax avoidance schemes, which show a steady decline as a result of tough government action. We brought in more than £1 billion from the first year of applying accelerated payments and have closed many loopholes and secured tough new enforcement powers.'