CP-22A's are often related to changes resulting from info provided by taxpayers.  Perhaps your clients have had some interactions with the IRS recently?

I have seen at least one CP-22A that had a very brief explanation on the first page.  If we hadn't prepared that amended return, we may not be able to make the head and tail of what that's about.

Agree it's probably the most expedient to call the PPS to find out what caused the increase in tax if your clients had not been in contact with the IRS (or received any prior notice).

The simplest way to submit a PoA these days is via the online service but the good old fashioned way of submitting PoA's with wet signatures by fax still works:

https://www.irs.gov/tax-professionals/submit-forms-2848-and-8821-online

https://www.irs.gov/individuals/understanding-your-cp22a-notice 

and they tell you exactly what to do and how to do it!

Thanks for replies. The notice is in response to a 1040X. The tax calculated by IRS does not agree with the tax calculated on the 1040X.

You would think they would offer an explanation..

I suppose you have already checked that the difference is not interest charges, the calculation of which should be detailed on the CP-22A.

Pull a transcript and see what's been changed. 

If you still can't figure it out, get a PoA and call the PPS.

Not the interest. It is the increase in calculated tax due.