I happen to be going through the IRS CPxxxx notifice process.
I got a CP504 in June (via certified letter) re: my '14 return.
It was so far off from my '14 return #s that I thought it was a scam. I confirmed the 800# on the letter was legit before I called it.
I strongly recommend a headset when calling. And, has been said, have the notification letter and all your #s available. And I agree with being nice and grateful on the phone.
On my first call, the lady confirmed who I am, put me on hold a bit and then said I shouldn't have gotten a CP504 notice. She put me on hold a while so she could (try to) confirm why I got it and/or what I should have gotten. She couldn't figure it out and said she/they would send me another letter when they figured it out.
I got a CP2000 in July (not certified).
I had sold some stock that year for a loss. They showed the sale (as from the 1099B), but somehow had simply tossed / ignored the base price and therefor the loss. They had re-calculated my return as if I had somehow gotten the stock for free and made the whole sale amount as profit/gain. Quite different from my return I actually 'filed' (mailed in).
I went thru my '14 return with a fine tooth comb and found a typo (swap of two adjacent digits w/in a number) that resulted in me over paying.
I called again, and had them (another lady) give me clear instructions on how to address both the base price / loss of the stock sale AND the amendment of my return for the typo.
It was actually more straight forward than I had feared. The hardest part was finding the 'fill-in' forms (instead of download to print forms) to download. Of course, I went over every line and digit 3 times before sending them.
The IRS gave me more than enough time to compile and send my 'reply'. You do have to send back one page of the notice with the reply.
I'm kind of eager to see how much interest the IRS pays me on my over payment.
Oddly; I got a CP90 notice (by certified mail) in Sept.
It was basically another CP2000 that I got in July, but the amount I owed had gone up because of accumulating interest charges.
I called again. They confirmed that they had gotten my 'reply' to the CP2000 from July. They said it can take up to 15 weeks to hear back. I got a canned answer when I asked why I got another notice after they had gotten my reply to the previous notice. She said she put the CP90 on hold.
Oh, have pen and paper (or computer) when you call. Each IRS person will give you their PIN (Personal ID Number?). Write it down, and all the key points they say, in case there is any question or problem later.