Each year, the IRS selects tax returns for audit at random, based on discrepancies in income reporting or on their Discriminate Function System (DIF) scoring.
An audit can either be conducted via the mail or in person. If your audit is conducted via mail, the audit notification letter you receive will usually contain the name and phone number of the audit examiner conducting your audit. With in-person audits, you're usually made aware of a tentative audit result before you leave the meeting with your auditor and are usually mailed the official determination after the audit.
If you are unable to get in touch with your audit examiner or have questions prior to your audit, call the IRS Customer Service line at 800-829-1040 between the hours of 7 am and 10 pm.