The IRS wants to help the small business and self-employed communities determine whether an unsolicited contact is truly from an IRS employee.
The IRS initiates most contacts through regular mail delivered by the U.S. Postal Service. However, there are special circumstances in which the IRS will call or come to a business, such as when a taxpayer has an overdue tax bill, to secure a delinquent tax return or a delinquent employment tax payment, or to tour the business as part of an audit.
See: How to know it’s really the IRS calling or knocking on your door for more information.