Default [Q] Route calls through second Android via internet

Here's the endpoint - I want to have an internet connected device in my hand that can make and receive calls and send and receive texts, regardless of whether I have mobile coverage. As long as it's on the internet, I want to be able to use it as a regular phone.

I'm halfway there.

I have an Android phone with my primary number, plugged in and connected to WiFi in my flat, a good mobile signal area. I can install MySMS on it to handle texts, then the tablet app on the device I keep with me. That's SMS sorted.

What I need is some way of making a voice call over the internet to that handset, and routing that out via my cellular provider. And automatically routing incoming calls over the internet to the portable one, or if I'm making a cellular call, have the caller ID display the number of the phone in the flat.

Basically, I think I want to turn an Android phone into a Google Voice server, just for my use.

Can this be done?