> It should not be a real phone. i.e., No baseband or actual cellular connection That's going to severely limit the usability of the device. If you want to mak

> It should not be a real phone. i.e., No baseband or actual cellular connection That's going to severely limit the usability of the device. If you want to make a device for practical security, you'd need to find a way to isolate the mobile network chip. IIRC, better USB 4G dongles are standalone and expose expose either serial interface for PPP or an USB ethernet device, so the attack surface is limited. If you build the phone around the chip from one of these, and make sure all data is VPN'd (and voice is VOIP'd), I think you should be able to contain cellular/baseband related attacks (to be clear, the baseband is still vulnerable, but won't be able to make the jump in the phone proper, and will only be able to see an encrypted data flow).