#1: Always send mail to them certified or registered.
#2: Scan or photograph everything. Always.
#3: Call them (but budget for a LOT of time on hold)
We had something similar happen. I was on hold for 45 minutes before someone came on and told me he had to transfer me to a different “accounting” department. Accounting department?? Isn’t the entire IRS one big accounting department?? An additional 90 minutes later I finally talked to the right person. 10 minutes later the issue was resolved. This was after 6 months of their letters, threats, extensions so they’d review my responses, etc.
If you don’t feel you owe the money, DON’T PAY THE MONEY! ...but definitely do your due diligence to document the hell out of everything, and call them.
We’ve called multiple times. The system is so full it won’t even put you into a call queue!! It just says “Call back later or the next business day.” That is all we ever get.