A friend had nearly the same problem. He has a Sprint HTC Diamond, too, and he just stopped the call when he pushed the answer button. Tried to help him solve the issue but couldn't figure out how to do it. Well, years ago I worked for a
telephone answering company and they were producing kind of pocket answering machines. A similar problem occured with them (couldn't "answer" call when you were at home, the machine did it instead), this was resolved by our IT deparment, was just a little bug in the software.