Thanks for contacting TextNow Support!
Your phone definitely shouldn't be so hot you can't touch it. You can check out the Battery section in your main Settings app to see if there's a particular app or service that's using the most amount of battery; closing or deleting that app might solve the issue.
A more radical solution would be to factory reset the phone to restore it to it's original state. If it's a software issue, then wiping the phone and re-updating everything might fix it as well.
And finally, it could also be a physical problem with the phone itself. The S6 doesn't have a removable battery, unfortunately, so if it is a problem with your phone we will replace it under warranty.
If the first suggestions didn't work, please give us a call at TextNow Support (from another phone or device, of course) and our agents can help narrow down the problem.
You can reach us at 1-888-250-5255, Mon-Fri 9am-10pm and Sat-Sun 9am-5pm, Eastern Standard Time. We look forward to speaking with you!