Find out what you need to do if you have lost or misplaced your Employer Identification Number (EIN). Employers withholding income tax from employees in Arizona must register their Employer Identification Number (EIN) with ADOR to file withholding returns.The only two ways to get an EIN Verification Letter (147C) is to call the IRS at 1-800-829-4933, or to mail the IRS a written request. Click the Sign Up button in the top right or bottom center of the portal. ➢ On the Sign Up screen, complete the required fields. If you have a Tax Identification Number, also called a FEIN number (Federal Employer Identification Number) you should use it when registering your company. How to Contact the IRS for EIN Verification Letter? The State of Arizona uses "The Work Number" to provide automated employment and income verification for our employees. Search our website for available business names and resources to help you through the filing process. Steps to verify EIN Number: Call IRS support.