you're wright, however you only can hear people, you can't reply !!!! I was thinking of this solution, however, I needed to install some pre-recorded messages, in electrical-board, then control messages with X10 (like for the water control), for example with some buton on the screen !!
However, your camera is expensive and I think a VOIP-intercom would be greater (but expensive too). I was thinking about a SIP voip cell phone, then sold wired for header and mic (the phone would be hidden).