[mirimir]

Well, then you get proper hardware.

[qb45]

Where are you sourcing those "proper" cell phone radios which don't contain secret binary blobs? I want one too.

[mirimir]

You nuke the radio, and use a standalone cell modem. The cell modem may be compromised, but it can't touch the device that's handling encryption.

[narrowrail]

Speaking of which, where is the flip phone with integrated 4G modem and wifi that can be used as a hotspot?

[plugnburn]

Why doesn't a cheapest Android smartphone with a 3G modem (say Alcatel 4009D) fit?

Because it's not 4G? Well, here's the real tradeoff: security, comfort, cheapness. Pick two.

[narrowrail]

Mostly because I believe the device can be more cheaply made if it does not have to run a full-featured OS such as Android. No browser, no color screen, just like my old Nokia candy bar phone; no GPU at all would be required.