idea: connect a Beaglebone Black single-board Linux computer to a SM5100B GSM radio module mounted on an evaluation board.

This device runs a VPN relay (using your home internet connection as uplink) and a server which relays GSM audio and SMS messages.

Next, delete the SMS and voice-call apps on your phone, replacing them with some yet-to-be-determined app designed to work through the relay server above.

Configure your phone’s internet connection to use a VPN, routing all data traffic – which now includes all voice and SMS traffic – through the relay box at home.

This allows you to run a firewall on the relay box which can whitelist or blacklist anything you want. Worried that your apps are phoning home behind your back? Block ’em. You could have different firewall rule sets, like “allow nothing”, “allow email only”, “allow email and these web sites”, “allow everything but block known malware sites”, etc.

Further idea: take the SIM out of your phone, put it in the relay box, and cancel your data plan. Buy a prepaid SIM with cash and put it in your phone. Now people who know your phone number and have the ability to track phones can see that your phone is sitting at your house, 24/7, but unless they know about your prepaid SIM, they can’t track your physical location. Bonus: travelling internationally? All your web sites continue to think you’re logging in from home, and you don’t have to worry about geo-restrictions.