Do you have proof that you paid? Your only proof is your own credit card or bank statement.

doesn't the notice provide a means to object to the change other than trying to call? i would send back the notice with an explanation, and support if available, as to why the IRS is wrong. keep a copy of what you send and keep trying to call. sadly the IRS is receiving more calls than it has phones and agents to answer. another possibility is to try to contact a local IRS office to set up an in-person conference. again multiple attempts may be required.

Is the IRS saying you didn't pay or you owe more? How did you pay? Did you mail a check with a 1040-V voucher or use a 1040ES slip? Your only proof is your bank statement or credit card showing the payment. If the IRS changed your return how much more do you owe? A common adjustment is if you claimed the Recovery rebate credit on 1040 line 30 when you already got the first 2 Stimulus payments or got the second one after you filed or reported the wrong amounts or entered them in the wrong box. Do you have an amount on line 30?