After you file

I received a Letter saying my 1040 stimulus miscalculation as well. I paid $975 in taxes and they are saying I now owe $2,423. The letter states If you agree with the changes pay it. It offers no way to review what the problem is. I just received the letter today 9/13 and the payment is due 10/4. Is there someone I can talk to about this. If I call the IRS number I get nowhere its just a bunch of recorded menus without the ability to get a hold of anyone.

 

Thanks Greg