Taxpayer Authentication Number
A ten-digit number printed on a private collection notice. It is split between you and the agency so each can prove who they are.
Full Definition
A Taxpayer Authentication number is printed on the CP40 or CP140 notice that tells you your account has been assigned to a private collection agency.
It exists to make an unsolicited call verifiable in both directions. On CP140 the IRS describes the procedure: the agency asks you for the first five numbers of the Taxpayer Authentication number included in your notice, and "the private collection agency will then provide the last five numbers to complete the verification".
Read that as a test you are administering, not one you are taking. Anyone can ask for digits. Only a firm that has actually been assigned your account can supply the other half.
On CP40 the IRS describes the same mechanism more briefly, stating the agency will perform two-party verification using the Taxpayer Authentication number provided in your notice.
If you do not have the notice, you do not have the number, and there is nothing to verify against. The IRS route in that case is its own published list of agencies under contract.
What to do next
Keep the notice. The number on it is the only thing that distinguishes an assigned agency from a caller who has your name and the fact that you owe tax.
What this page does not say
- Neither the CP40 nor the CP140 page states what to do if a caller fails the verification. Publication 4518 has not been retrieved. Reporting routes for suspected impersonation are not described here.
Sources
- Understanding your CP40 notice — Internal Revenue Service, retrieved 2026-08-21
- Publication 4518, What You Can Expect When the IRS Assigns Your Account to a Private Collection Agency — Internal Revenue Service, retrieved 2026-08-21
Last reviewed 2026-08-21 by Tax Resolution Clarity editorial.
Sourced to primary IRS materials and editorially reviewed. Not reviewed by a tax professional. Not tax advice.
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Where to go next
The IRS publishes the rules this page describes. Your own notice governs your dates.