Is this a known problem? We have a couple of Cisco 7960's here that seem to have a bad 'hook'. When the handset is lifted, either with an incoming call, or to make a call, the phone doesn't always seem to notice this. Sometimes it doesn't respond at all, sometimes it disconnects the incoming call, and sometimes it works the way it should. It's like playing Russian roulette... :-/
The 7912 we have here does not have this problem...