If the caller is an authorized person, for example an owner, partner, corporate officer, trustee, or executor of an estate the IRS will provide the corporate ID, known as an EIN, over the phone through its Business and Specialty tax division. The appropriate phone number is on the IRS website.
EIN stands for employer identification number. This number has nine digits and is also available on past tax filings and bank account paperwork. If the corporation is publicly traded, then the EIN is also on the 10-K filings accessible on the SEC website. A company's Dun and Bradstreet report may also include the EIN. An owner who operates more than one business has a separate EIN for each business.
Most states administer a separate registration system and issue a state corporate ID number to applicants. In some cases the state corporate ID number is the same as the EIN. Many states provide a public database of state corporate ID numbers and filings which are administered through the office of the secretary of state.
There are also sites such as EINFinder.com and FEINSearch.com that cater to commercial customers but provide limited free trial searches. Prospective users must register on the sites before conducting trial searches.