Download a sample IRS Letter. Can't find your letter? Call the IRS Customer Service for nonprofit organizations at 1-877-829-5500 and give them your nonprofit’s name and Employer Identification Number (EIN). Do not upload State issued tax documents, a Form 990, or an SS-4. An SS-4 is a form letter sent by the IRS informing you that your organization or business has been assigned an EIN (Employer Identification Number). Unfortunately, it in no way confers tax-exempt status.