I have one that came with the house. It has 3 zones - one button calls my daughter in her bedroom and in the playroom next to her room, another button calls my second daughter in her bedroom and a small playroom next to her room, and the third button calls my bedroom downstairs. I have the main call box in the kitchen. A second call box is in my bedroom. The girls can talk to me from upstairs without having to get up and push a button, which they like - I have to be downstairs and push a button to talk to them.
I can also just hold the button in and listen to them upstairs, without them knowing I am doing it. Very sneaky but occasionally useful!!! (I only use this to find out if they are upstairs by the sound of the TV - otherwise I'm not really that much into eavesdropping!!!!)