The very first part of the article, "Hardware Selection", covers this point and recommends that the secure device not have a baseband. They use a wifi-only Nexus 7.
I really wish you could get wifi only phones. The Samsung galaxy player was a good candidate a few years ago, but that's quite old now and doesn't appear to be a trend that continued.
A wifi only phone with a full size USB and a very small GSM thumb drive ... that would be workable. Also: fantasy.
OR: a normal phone with a GSM sim module that you could add/remove quickly, like a SD card - without opening the device.
> What if we had a killer phone-over-wifi app, to allow this segment of the market to take off?
It's called VoIP. The carriers do everything they can to kill it (to keep you buying cellular service for voice), but it's not like the technology doesn't exist.
Yeah, the technology has been around for a while, but there were a lot of limiting factors (there may still be) for pulling somethings off that might be worth revisiting now.
Meta: My dad works on VoIP systems for BT and I was telling him he'll probably be on the front lines in a couple of years when mesh-networking (well he calls it a buzz word for peer-networking) takes off and efficient packet routing on decentralized nodes that could come on/go offline at anytime.
Facetime, as an example, is a voice-and/or-video protocol which works over wifi and is not trace-able/snoop-able by Apple (according to their recent document about their law enforcement compliance).
A wifi only phone with a full size USB and a very small GSM thumb drive ... that would be workable. Also: fantasy.
OR: a normal phone with a GSM sim module that you could add/remove quickly, like a SD card - without opening the device.