If I owed a total of $1629 Fed taxes, and paid $400 when first filing, then later paid $685 more, what do I do if I get 2 IRS notices saying I owe: $943.14 and $306?

1. I retrieved my "Account Statements" from the IRS website, and it clearly shows both my payments of $400 and $685, which confirms that the IRS received my payments.  This means that my balance should be ($1629-400-685 = $547, and not $943.14).

2. The $306 notice was for "unpaid shared responsibility for 2018", however, I included that in my taxes, in my tax return, therefore, I have already paid for it.

I have tried to contact the IRS, but am unable to do so.

I would appreciate any advice on how to approach this problem?