On the websites & google you will find several fix iPhone options but these are not actually working or they are coping each other.

To fix your missed calls notifications it is so simple. Follow the following steps & you will be notify when you are on call. I am using iPhoneX with iOS 13

Fix iPhone miss call notification

1. Go to iPhone Settings

2. Then go to “Phone”
   

3. Then go to  “Call  Waiting”  Calls  section

4. Then turn on Call waitingNow test. It will work. Enjoy