The bill you refer to is the bill for penalties and interest from 2017 and 2018?
Reply by letter to their request (which probably invited you to disagree as you said, giving you the chance to ask for the abatement) as soon as possible. Then send the reply by certified or registered mail or by UPS or FedEx (if you have a street address for that office), so that you can demonstrate later that you replied in plenty of time once that office starts to dig itself out and tries to figure out who answered and who didn't.
Please add a note in your letter that you tried to reply by phone as directed in the letter, but that the phone number was no longer accepting calls (they will probably know that, but it's good to remind them that you have been trying to abide by their request).
So you have not mailed the 2017 and 2018 state returns yet? Hmmmn, what you might do is in your response, ask them if they would like you to send the state returns (with the federal copies, of course) to their office. They will probably say, "no", but give them the chance.
And mail your 2017 and 2018 returns anyway, again by certified or registered post, or by UPS or FedEx.
As for the due date to pay, generally (I haven't seen your letter), the due date is to pay or respond...and you are doing your best to respond.
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