For some unfathomable reason there are various people that propagate the misinformation to others that our tax system is voluntary. In fact, as one might imagine, it isn't voluntary, and the Internal Revenue Code, makes it explicitly clear that the paying of taxes is actually compulsory, for all those generating income at a level in which the payment of taxes is required. Further to the point, if taxes were somehow voluntary, then those that are employed and being paid wages, would not have appropriate taxes withheld from each of their paychecks for various taxing categories, but rather would have a strong inclination to consider opting-out, per their volition. Additionally, people that are self-employed, in which their compensation is based upon an agreed amount between those parties to the contract, are legally subject to having a 1099 form issued to them, stipulating the amount so paid during that year; of which, the recipient of that 1099, has the full responsibility of paying the appropriate amount of taxes on such, in which the IRS is cognizant of the issuance of that 1099.
In a lot of ways, in a society in which most everyone has a Social Security number or a Tax ID number, any labor that is done, that is considered to be income, is subject to taxation, and it is far easier to track that labor and work, because of that identification having been recorded. However, in those jobs, in which cash passes from one hand to another, though it might well be labor, subject to taxation, the fact that the transaction was done in cash, truly does then make it "voluntary" as to whether such is properly recorded to the governmental tax authorities as income or not.
So then, perhaps when people state that we have a voluntary tax system, they mean that transactions that are accomplished, in the underground economy or similar, in which those transactions are done with cash or cash-like instruments, ends up being essentially voluntary as to whether or not someone reports that income and hence pays their appropriate taxes on such. This is most definitely true, and a very strong reason why governments all over the world are trying to reduce or eliminate cash transactions, for not only does the taxman want their fair share of the money, but this is also the means for the government to keep their finger upon the pulse of their population.
While it is true to state that the government does not dictate to its population how much that a given citizen owes in taxes, it is that same government that thereby determines as to whether or not tax returns so filed are considered to be accurate or not. So that, citizens voluntarily fill out their tax forms and therefore have control over what is thereby submitted to those authorities, in which, to a certain degree, those taxpayers determine the amount of monies to be paid to their government, but such is subject to the auditing power of that government, of which, that government is in essence, the ultimate authority as to whether such taxes have been fully and properly paid, as mandated by law.