Thanks to this video intercom you can answer anyone who push the video intercom button using your mobile phone and you can do it even if you are away from home.
In the kit is present also an internal bell that you can put every where in your home.
Here there is the manual.
The video intercom work only to WiFi at 2.4 Ghz and in the kit is present an appropriate power supply.
In any case is possible power the video intercom using an external power supply from 16 to 24 Vca (30VA).