If you don't qualify for an IA through OPA, you may also request an IA by submitting Form 9465, Installment Agreement Request, with the IRS. When you request an IA using the form, generally, you'll receive a response from the IRS within 30 days notifying you of whether the IA request was approved or rejected.
If you don't report income that you should have reported, and it's more than 25% of the gross income shown on the return, or it's attributable to foreign financial assets and is more than $5,000, the time to assess tax is from the date you filed the return.
Or: For individual tax returns, call 1-800-829-1040, 7 AM - 7 PM Monday through Friday local time. The wait time to speak with a representative may be long. This option works best for less complex questions.