I got a CP11

I sent my return and had to pay. I automatically set the payment to execute on April 14th. The payment was executed. I sent them 2130 and it cleared on the 14th. I now got a letter saying they recalculated my return because of the child tax credit I paid back, my situation changed I shouldn't get a child tax credit, and the letter says I owe 1630 plus 2 buck interest. from the 15th til the letter date.

I paid on time and by their calculation, I now overpaid 500 dollars. Can't get through on the phone, my online account says 0.00. How do you go about getting the 500 back?