I have the same problem!
I have the same problem. It started about 3 days ago for no reason. When you press the call button after entering a number, the screen goes black and all the three buttons at base (back, windows and search) are not responsive. There is nothing you can do...you can't hangup or enter numbers until the other party hangs up, unless you reboot. (You can still talk and make the call as normal).
I tried rebooting, took battery out and also did a factory reset but nothing has resolved the problem.
However, I did find a workaround:
flip phone over so it goes into speaker mode and then the dialpad becomes visible and you can end the call or use number pad, but you have to keep phone flipped over all the while. Another workaround is to use a Bluetooth headset when making or answering calls. Then the dialpad is always visible as normal.
If anyone knows what to do please respond!