There is several ways you could accomplish this.
Each person/device can get their own GV account and call anyone with the Hangouts app. Android has a separate app called Hangouts dialer.
Get a free Callcentric account. Each device would be assigned an extension at callcentric. Install a softphone on each device. Each device can call any of the others by extension number.
Get a Sip2Sip account for each device. Using a softphone a device can call the others by account number.
Use the OBion app. OBihai just updated the app for iPhone/iPad. The Android version hasn't benn updated since 2011 and has bugs for newer versions of Android.
I would choose one of the 1st two.
See this for other ideas:
http://www.obitalk.com/forum/index.php?topic=9351.0