Do not agree with the amount the IRS says I owe in the letter they sent but cannot get in touch with them to discuss

I got a letter from the IRS stating that I owe more money than what I paid with my return.  I do not agree with what they have determined.  It says that it is due on 4/18/22.  It also says that if I do not agree, I should call them, but I have tried calling the number they provided multiple times and have not gotten anyone on the line.

 

Since it is so close to the due date, I am not sure if I need to pay it and then try to get them to refund the money later or if I should not pay and try to get them on the phone after the tax deadline when they may not be as busy.  But if I do the later, I am concerned that I will get penalties added.

 

Thanks for any advice with this matter.