If you've been contacted by the IRS, note which type of agent called you. If it was a special agent, then, you are probably being subject to a preliminary investigation. They may have alternatively identified themselves as part of the IRS Criminal Investigation or IRS CI.
If it was a revenue officer, they were calling about an unpaid tax or unfiled return. If it was a revenue agent, then, they were probably contacting you about an audit. Both of these IRS employees can refer your case to a special agent, but if they're still calling you, then they likely have not referred your case for an investigation.