IRS Form CP80

I received IRS Form CP80 for my electronically filed tax return for 2019.  It is now November 2021.  I  have proof that the return was electronically filed and accepted, but they apparently have no record of the return.  The IRS has a $900 credit on my account and says it will be lost if I don't send printed copy of return.  I can't get through on the phone.  What's my best recourse?