Old post, but I'll mention that MOST dumbphones today use a 1/8" tip-sleeve (mono) connector instead of the 1/8" tip-ring-sleeve (stereo) 3.5mm cable you'd use with headphones. From personal experience, my previous two phones required a resistive load on the tip, and trying to short the ring with the tip into a stereo caused the phone to mute itself. I believe this is a TTY accommodation.