After you file

I filed my state and fed on time. I owed around $8k on fed and requested a payment plan. I received in the mail a notice that I was going to start being deducted the agreed amount. All fine.

I then got a second notice from the IRS saying that I owe $8k plus late penalties and interest and that I need to pay by 6/24/2019. I called the IRS and the automated system said that I was charged for failure to pay estimated taxes. I need to know what this notice means, does it mean that I owe an additional fee or is this part of the payment plan that I agreed to. I do not want to incur additional penalties on my balance. Is there someone I can talk to over the phone about this? Thanks.