that that or just use a very simple phone with no camera or microphone , keep it in a soundproof shielded box when not in use
You need a mic to be able to speak on the phone. You could plug one in for incoming calls, but you need one or the phone is useless. This process describes my hobby laptop. I can manually add sensors but as I type this there are zero.
I reckon my 1st smartphone will be one of these..... http://www.davidhunt.ie/piphone-a-raspberry-pi-based-smartphone/
Oh Yes ! I want that one . That easily trumps my previous notion of an " interesting " secure phone - https://www.wilderssecurity.com/threads/unaphone-zenith-secure-and-private-smartphone.385524/ That one might be a smart phone , but it is certainly a stupid price