Why is why phone heating up when making calls?

[Windows Central Question]

On my Nokia Lumia 635, I've noticed that my phone has been getting hot quickly while making calls and I am not such why.

 

[Laura Knotek]

Hi and welcome to Windows Central.

How is your coverage in the area where you are making these phone calls? If the signal is low, then there will be more heat due to the device struggling to stay connected to a tower with low coverage.