I have been lucky enough not to have to deal with one of these notices, but I found the IRS explanation of it online at https://www.irs.gov/individuals/understanding-your-5071c-notice .  What it seems to say is that you can create an account online (the 2848 should allow you to do this) and answer questions without calling.  Or, if you want to call, 

"Note: An authorized third party can represent you and call us on your behalf, but we must have a completed Form 2848, Power of Attorney and Declaration of Representative PDF, on file for the representative. Otherwise, you and that person must call us together and you must participate on the call."