The voluntary part only speaks to the fact that you “are responsible as an individual taxpayer to calculate what you owe. You’re expected to voluntarily comply with the tax code by reporting what you owe to the government and paying the entire amount that you owe under the law.”
“Although the U.S. tax system is voluntary, failure to comply carries stiff penalties,” they note. “The IRS also has the power to levy your bank accounts, garnish your wages and place a lien on your property if you don’t voluntarily pay what you owe. In serious cases, you may even face criminal charges.”
Lysander Spooner, in “No Treason” written from 1967-1870, writes of the notion of voluntary taxation, “this theory of our government is wholly different from the practical fact. The fact is that the government, like a highwayman, says to a man: ‘Your money, or your life.’ And many, if not most, taxes are paid under the compulsion of that threat.”