Phone calls naturally cause a battery to work hard and when they do, they generate heat. If your phone was actually overheating, it would display a graphic of a thermometer and tell you the phone cannot be used until it cools to a normal temperature. Since you're not seeing that, you're phone isn't actually overheating. But it is getting warmer than you expect. If you're using cellular and the signal is weak, your phone will work harder to remain connected, which could contribute to the heat. Your battery may be starting to fail too. A diminished battery will certainly cause more heat during activities which are intensive, such as making a phone call, using the camera, streaming, playing video games.
Check the health of your batter, by opening Settings > Battery > Battery Health - if it is at or below 80, the battery in your iPhone needs to be replaced.