If IRS Criminal Investigation has contacted you, you are no longer dealing with a bill. You are dealing with special agents whose job, under IRM 9.1.1.2, is investigating potential criminal violations of the Internal Revenue Code and related financial crimes. What you say from here forward can be used as evidence.
What CI investigates
The core tax crimes are in the Internal Revenue Code: tax evasion under section 7201, willful failure to collect or pay over tax under section 7202, willful failure to file or pay under section 7203, and false returns under section 7206. Each one requires willfulness. That single word is usually where the fight is. Read the criminal tax statutes overview for the elements.
Privilege is the reason to call a lawyer first
Your communications with your lawyer for the purpose of legal advice are protected by the attorney-client privilege. The separate practitioner privilege in IRC 7525 does not apply in criminal matters, which is why who you talk to, and in what capacity, matters. See Kovel arrangements.
Timing
By the time agents knock, they have often been working for a while. CI cases come from fraud referrals, other agencies, private parties, and CI's own projects, as the Justice Manual describes in JM 6-4.110.
Voluntary disclosure has a deadline
The IRS Voluntary Disclosure Practice only accepts timely disclosures. Once an investigation has started, the door is generally closed. Read the Voluntary Disclosure Practice guide.
General information only, not legal advice. Reading this page does not create an attorney-client relationship. Laws and IRS procedures change; the sources cited were reviewed on the date shown above.