A dumb phone might not be much better. There wouldn't be a camera for them to get, but they might still be able to get the microphone.

Many dumb phones have off switches (which actually work), or better: removable batteries.

(You can simply break the circuit with a piece of paper between the battery terminal and phone contact for convenience.)

Maybe fewer avenues for them to activate the microphone, though.

It has always been, because they were never open systems. I'm waiting for when it isn't socially unacceptable to ask people not to bring their phones when they visit.