Being frivolous, not answering the question, but for my money, the best dumb phone is the analog one we used to have, plugged into a socket in the wall. It was

Being frivolous, not answering the question, but for my money, the best dumb phone is the analog one we used to have, plugged into a socket in the wall. It was often noisy, sound was crummy, it was unwieldy, and it cost a small fortune to run, but it was without perceptible delay, it was fully duplex, and it far better approximated real face to face conversation than the weird false starts and constant stuttering interrupts we have learned to take for granted these days. Also, you could walk out the door and ignore it.