Android phones, apps and tablets

Yes, it's a nice idea but I think you're probably better off with a mifi, a phone that can do voip over wifi and an elastic band.
Maybe also one of those big batteries with micro-USB to power it all.
What you then probably want is the mifi to autoconfigure a VPN to home or to use something like the Mobile IP stuff in IPv6 which I've failed to find usable open source implementations of and which looks even more of a headache than IPsec.
So, basically, a mifi running OpenBSD or something.

For this sort of thing, you might want to look at someone like Arnold & Andrews, who might be able to help. Pricey, but very good at what they do (ISP and telecoms). I think that they have things like this already setup, although that may only be for their MD.