WiFi communication across LAN

I apologize if this question has been asked before. My parents have a WiFi network setup between their house and their rental house next door. I am trying to find a device that would work via WiFi to allow voice conversations. My sister lives in the house next door but neither her nor my mom are good with technology. Using a messaging client on a computer is not an option as neither of them would be able to operate it. I was hoping there might be a handheld device or even a device that connects to a computer that would essentially allow them to "telephone" each other. It needs to "ring" or have some form of notification. Any ideas? Thanks in advance.

Matt

Skype

I already thought of Skype. I was looking for something that might work internally on the LAN and would not require outbound/inbound communication through the Internet. I guess maybe I have stumbled on to a potentially new product. What I want is an intercom system that works via WiFi with a notification system. I'm assuming it doesn't exist.

Skype

WiFi phones have an intercom feature. They can call each other in network without going to the internet.

Takara's VoIP intercom phone - Engadget

Zyxel P-2000W V2 - VoIP Wi-Fi Phone

ZyXEL

Google wifi phone intercom