How do I find out if my tax paper filing documents have been received by IRS?

For the tax year 2020, I received a filing extension to 10/15/2021. I mailed my tax forms on 10/11/2021. I have looked on-line at a Verification document and it reads " we have no record of a processed tax return for the tax period December, 2020". I am trying to find out if the documents were received by the IRS? Perhaps due to backlogs, they have not been processed yet. I have tried to get through to the IRS at the 800- -1040 number but get to a point at various times where I will be sent to the proper group, but when I am transferred I get the recording "..to busy now try later". Rather than a refund, I applied to have my refund applied to my 2021 estimated tax. I have been advised by IRS personnel not to send in a copy of what I have mailed and not to send a letter asking if the IRS has received the return. Any suggestions on how I might get this information?