My iPhone 6s plus is heating up while talking

If your iPhone 6S plus heating up when talking on a call but works fine when using it with a speaker? Know from our experts what might be wrong with your iPhone and what you can do to fix this.

I have one iPhone model 6S plus. It is getting heated up while using it especially in talk mode. But when I am keeping the speaker in on position then it is not heating up. Has anyone encountered such a problem. What could be the probable cause of this problem? Experts may share their views and experiences.