Hi all,
sorry for being a bit off-topic here but I think if you still have a dump phone, not using the Internet and only GSM networks and an additional offline mini notebook, you will probably love the idea of Red Phone. I hope this thread get’s not deleted by moderators, because I think this is quite cool. This project is inspired by the famous Navajo Code Talkers, which nobody could understand, and all cool spies stuff from the cold war and EU politicians who have AES hardware based encryption mobile phones, which are not available to the general public. I think we should have these secure capabilities too. 
I hope you like and enjoy Red Phone!