Suppose you made 10 trips to the ATM, but three of those times it was out of order. Or if, in 10 visits to a fast-food place, the milkshake machine was broken three times. Think you’d find a different bank and a better take-out restaurant?

The fact is, when dealing with customers, getting it right 70 percent of the time isn’t nearly good enough. Unless, of course, you have the power of the U.S. government behind you.

As Americans begin thinking about tax season, the Internal Revenue Service promises that, this year, it will aim to answer 71 percent of the phone calls to its help-line number, 800-829-1040. So the agency begins the tax season by admitting it plans to ignore three out of every 10 callers.

This would actually be a slight improvement from 2008, when only five out of every 10 calls to the IRS were answered. And it would roughly equal last year’s “success rate,” when seven out of 10 calls were answered.

Meanwhile, those who do manage to get their call answered must endure the waiting game. The average caller will spend 12 minutes on hold.

“This level of service is unacceptable,” National Taxpayer Advocate Nina Olson reports. She called the IRS’s unresponsiveness the “number-one most serious problem for taxpayers.” (Wonder if she’s ever been audited?)

In the agency’s defense, spokeswoman Michelle Eldridge told The Washington Post that, “The bottom line is we have answered millions more phone calls in the last two years than ever before.” That’s undoubtedly true. Year after year, Congress makes changes to the tax code, making it even more difficult to understand. That’s why most taxpayers pay someone else to prepare their return; they’re afraid to make an expensive mistake.

They’re right to worry. Even if they can get through to the IRS for help, taxpayers are often steered wrong.

For example, a 2004 Treasury report found that the IRS provided incorrect answers 20 percent of the time. In the agency’s defense, that may be because there often isn’t one single, apparent correct answer. For many years, Money magazine asked 50 tax professionals to prepare sample returns. Year after year the pros came up with different tax liabilities – sometimes differing from each other by as much as $1,000.

Even when IRS advice isn’t wrong, it’s often misleading.

Last year, the Treasury Department studied the IRS’s program that provides volunteer tax preparers to those who ask for help. Some 41 percent of returns were filled out incorrectly. “If 17 of the incorrectly prepared tax returns had been filed, taxpayers would not have received $4,138 in tax refunds to which they were entitled,” the Treasury report found. Quite a price to pay for free help.

The real problem is that lawmakers keep trying to use the tax code to shape social policy. They then compound the mistake by ramping up the IRS workload without providing additional resources to do the work.

Congress gives tax breaks to induce people to save for retirement – but caps those breaks so people won’t save “too much.” It aims to encourage oil production – but in trying to discourage energy tax shelters, lawmakers thwart most of the incentives.

It gives tax breaks for installing a particular type of window in your home. It gives a tax credit for each child, but beware: the amount of that credit will plunge next year unless lawmakers act this year.

Enough. The purpose of a tax code is to collect revenue to fund the federal government. And that’s all it should be used for. In 1986, lawmakers greatly simplified the tax code, but we’ve allowed it to get out of hand again in the decades since. It’s time for a flat tax that we can all understand and comply with.

Maybe we can’t force government bureaucrats to become as responsive or effective as McDonald’s employees are. But at least we can ensure that Americans can fill out something as basic as a tax form without needing to place an unanswered or errantly answered call to the IRS.