If the phone is overheating then it would show you this via a pop up and advise go turn gne phone off for it to cool down or it would shut itself down @Pippy361
Are you seeing this pop up ?
The phone however shouldn't be uncomfortable to be used against your face while within an active call.
A phone will warm up depending on what is being performed on it and 2hat it's doing in the background.
Apps like CPU-Z and phoneINFO from the Playstore will help show what CPU processes and what temperature the phone is running at.
Please try these first and if these don't help please use a Samsung Experience Store / Samsung Service Centre for support.
Current Phone > Samsung Z Fold³ 5G. > Software : Android 12 ~ Samsung One Ui 4.