I agree with others here. It's the holster pressing the "End" key on your phone that is hanging up the call. Get an open face case seen at these sites
Open Face Case for BB 7100i.
Krusell Open Face Case for BB 7100i
I love this type of case for the simple fact that you never have to remove the phone from the case to use it. I have one on my 7105t and am soon ordering one for my 7100t. One thing about this case (and a function that I don't really need) is that it lacks the ability to put the phone to sleep and turn the screen off, resulting in battery drainage but I have 4 batteries to cycle out plus I keep my BB connected to my pc to charge so it's not an issue for me right now.
Note: They are the exact same case but sold by 2 different companies.