Taxpayers who received a notice in their mail that they must fill out a form to request a tax identification number (TIN) cannot be contacted. To report tax fraud and errors you can contact the Idaho Income Tax Division. If you need assistance with a state tax matter, call the Idaho Income Tax Division at, and we'll help you. Form W-9, Request for Taxpayer Identification Number- IRS Idaho Form W9 and TIN Application Instructions. Form W-9, Request for Taxpayer Identification Number What is the tax Identification Number (TIN)? The tax identification number (TIN) is required of all persons filing a tax return or who are required to pay an excise tax. Tax returns are also filed electronically with the Idaho State Department of Revenue's (IDOL) Income Tax Refund System. What does an IRS W-9 do for you? I-844T (W9, W-9) The form asks you to fill out the first eight lines of form. You can use your phone number to get help to fill out the form. Form W 9 (W-9) (Request for Taxpayer Identification Number) Form W-9 (W-9) This is used when you make an investment. In most states, people can't be reported as co-invested income because they aren't listed as investors in the state's income tax returns. An individual's “investment income” isn't taxable. Its money earned from investments such as stocks and mutual funds, that has an associated profit but doesn't yield income. For this and many other reasons, the IRS prefers to report investment income from businesses to the government, even though there is no direct profit from the investment. The other way to report investment income is to report your personal investments, which are part of your tax return, to the IRS at that time of year. It will be reported on your federal income tax return as an “investment income” in the following tax year. This should be the month that you invest. The W-9 can also be used for small business entities to report investments and losses.