How do I fix missing 'accept or reject' pop-up for incoming calls on iPhone 8 Plus?

Try a forced restart.

• Press and quickly release the volume up button

• Press and quickly release the volume down button

• Press and hold the side button.

• When the Apple logo appears, release the button.

Force restart iPhone - Apple Support (IN)

"How do I fix missing 'accept or reject' pop-up for incoming calls on iPhone 8 Plus?: Phone rings but there is no way to answer the call the phone rings but the pop up that says accept or reject does not appear and the caller does not appear in recents - how do I fix this? This is my wife's iPhone 8 plus"

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Seeing and Answering Incoming and Outgoing Calls:

A. Remove Security Software:

See if there is any security software filtering these calls. Is there any security software on your phone? If so, remove it, as it just gets in the way on an Apple device, and this is a pure example how and why.

B. Make "Phone: Default Calling App:

1. Go to: Settings
2. Tap: Phone
3. Tap: Default Calling App
4. Tap: Phone

C. Full-Screen Incoming Calls:

Make these incoming calls go to a full-screen, rather than a banner.

1. Go to: Settings
2. Tap: Phone
3. Tap: Incoming
4. Tap: Full Screen