I have it and used it in the past. It’s better in privacy than Signal since it’s decentralized and doesn’t use any identifiers. Signal forces you to use the worst identifier ever: your phone number. It should be optional (as Threema) for easy contact discovery, not mandatory. Back to SimpleX, I stopped using it because every time you have to connect the computer to phone if you want to use it from computer (it’s for maximum security, but inconvenient for me). But it’s a great app.

Yes, sim calls and sms are not private, both on dumb and smart phones. They also connect to cell towers so your location might be found. Anyway, I think the hardware and software of a smartphone is more capable of surveillance you than a dumbphone. The only realistic way is to leave the phone home and keep a paper list of your contacts at you. And call them from a stranger if you need to. I did this and the biggest inconvenience is that I can’t take pictures or videos (I might buy a camera, I don’t know if they have location system incorporated).