Good morning khushdeep777,
Welcome to the Apple Support Communities!
I understand you are having trouble answering calls since you updated to iOS 11.3. There was a subsequent update for iOS 11.3.1.
I would suggest updating to iOS 11.3.1, restarting the iPhone, and then checking to see if there was an update with your carrier. Click on the links below for complete details.
Update your iPhone, iPad, or iPod touch​
Get help with over-the-air iOS updates
Remove and download the iOS update again
​
If you still can't install the latest version of iOS, try downloading the update again:
- Go to Settings > General > iPhone Storage.
- Find the iOS update in the list of apps.
- Tap the iOS update, then tap Delete Update.
- Go to Settings > General > Software Update and download the latest iOS update.
If you don’t see the update in the list of apps or if the issue happens again, you need to update your device with iTunes.
Update your carrier settings on your iPhone or iPad
Take care.