Hi _shree,
Welcome to Apple Support Communities. We understand that you're having an issue with the display on your iPhone 12. We'd like to help.
After this line appeared on your device during that FaceTime call, did it stay on your device after you ended the call? Did you get a chance to restart your device? If not, let's have you do that now:
How to restart your iPhone X, 11, 12, 13, or later
1. Press and hold either volume button and the side button until the power-off slider appears.
2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.
It's possible that you may need to have your device evaluated to confirm if this is a hardware issue or not. There are a few options available to get that done, according to the country or region you're in. Check out the locations here to get an idea of what is available to you: Find Locations - Apple
Let us know if that helps.
Cheers!