I'm certainly no expert on this, but it is a common issue apparently. I've never had the problem.....
I would reccomend reading throught this thread....
7100t Shuts off During Phone Calls
If you decide that you need to downgrade the OS, the link is here.....
10 Steps to Downgrade to 3.8 from 4.0 OS on 7100
I am NOT reccomending this....I am just providing the information I have seen on here before.....
Unfortunately, I'm off to a meeting. sorry I couldn't be more help!
Good Luck.