Delay in action and response time of my iPhone

When I make a call, the iphone looks like it’s not performing any action, but in the background the person is getting connected and I will be able to get the response only after 15 seconds.

The same happens while a receive a call as well. I will be able to hear the person in call only after say 15 seconds, after having attended the call.

Any solution or fix for this issue. This is really frustrating and I am thinking me to go back to android.

[Re-Titled by Moderator]