The Government Accountability Office has released IRS Faces Challenges Providing Service to Taxpayers and Could Collect Balances Due More Effectively (GAO-13-156):

While there have been efficiency gains and efforts to improve
service, the IRS faced challenges providing
telephone service and responding to correspondence, continuing trends
experienced in recent years. In 2012, 82 percent of individual taxpayers
filed their returns electronically (e-filed), reducing IRS's processing
costs. IRS also increased calls answered using automated service and
added a variety of self service tools, which helped gain efficiencies.
However, IRS's level of telephone service (the percentage of callers
seeking live assistance who receive it) declined to 68 percent. In
addition, of the 21 million pieces of paper correspondence IRS received,
about 40 percent were considered overage (meaning that IRS did not
respond within 45 days of receipt), an increase compared to last year.
While IRS plans to continue to pursue efficiency gains, its strategy for
future years does not specifically address how it plans to reverse
these negative trends. Reversing the declines in telephone and
correspondence services may require IRS to consider difficult tradeoffs,
such as reassessing which phone calls IRS should answer with a live
assistor and which it should not because automated services are
available.