Hi vinil_kumar,
If you are having issues with intermittently showing vertical lines on your display during calls, you may want to try some things to troubleshoot.
First, quit all running applications and test again -
Force an app to close in iOS
Next, I would try restarting the iPhone -
Restart your iPhone, iPad, or iPod touch
If the issue is still present, you may want to restore the iPhone as a new device -
How to erase your iOS device and then set it up as a new device or restore it from backups
Thanks for using Apple Support Communities.
Best to you.