Hi Swmr21,
Thank you for using Apple Support Communities. From our understanding, you are seeing a grey screen on your iPhone when you receive a call and are unable to answer the call, is that correct? When the call is missed, does your iPhone go back to the home screen or is it frozen on a grey screen? Has your iPhone been restarted since noticing they issue?
If the device is frozen, try the steps listed here: If your iPhone, iPad, or iPod touch won't turn on or is frozen.
You'll also want to make sure your device is fully up to date ( latest version of iOS 14 is 14.4).
Also, are you able to make calls without an issue?
Let us know!