Additional to what Giulio says, it is normal for your phone to become warmer when it is working hard. A video call gives the processor a lot of work, the screen is always active, and then all the data has to be sent over the cellular radio. All these things use energy and generate heat which has to be dissipated through the phone’s body. Your hand is probably around the phone to limit heat transfer, and if you have a case on the phone it also reduces airflow over the phone which would carry the heat away. As a rough guide your phone can become warm, but it should not get to the point where it is uncomfortable to hold, and it should never become too hot to touch.