What is the temperature of the room?
Where did you get that number - 120 degrees - is that battery temperature
Where I am right now it is 77 degrees and my V10 is resting, battry temp 87 degrees which is normal.
Things like syncing, backup, system update, antivirus scanning etc can cause the phone to heat up when you'r not using it.
Like B. said, a poor LTE /cellular signal can also heat up the phone as it tries to connect.
If you can give us more details we can help determine if it is normal or if the phone is bad. Some phones run hot due to poor heat sink on the CPU for example. THe solution to that is replacemnt.