We bought an intercom set from Radio Shack. You just plug them in the wall and they work really well. My dad uses the same thing between his house and his workshop which are separate buildings on the same lot and it works great for them also. Our has a call button and a talk button. It is even expandable, so you can buy another set for other rooms. The call button has a loud enough beep/tone to wake at least one of us up in the middle of the night. You can even lock it on so they don't have to press a button at all, but of course then you can hear every sound from the other end. You can even use it on battery power if you want to go sit outside and still hear. This link should get you to the product page. We didn't buy ours online, we picked it up in the store.
Hope this helps,
We also use the system from Radio Shack that plugs in and uses your house electric current. It works gets except when Ryan tries to reach for it and it falls on the floor. Then he usually has his service dog bark to get my attention. Like the other post said, you can lock it so it then acts like a baby monitor. It doesn't have as much static though. We have one in every room of our two story house even to get the attention of our other children. We love them.