Good luck, dumb phone means no encryption, SMS and MMS are stored plaintext and freely shared with US three letters and phone convos have been spied on en masse since the 80s.

For that to be effective you’d have to give up phones, smart, dumb, even carphones and landlines. Not only that, you’d have to reliably know your contacts are never contacting you through their phones, windows PCs, etc, that all themselves have that compromised HW as well. Realistically you’d need to fall back on the postal service, but people can steam open letters, so you’ll need directly employed couriers, but can you really trust anyone else? So you need to hand deliver it, but can you really trust your contact to burn after reading? Better say it face to face, if you can be sure they (or third parties) aren’t recording, but then Flock knows they were with you and they could be tortured later.