I purchased a iPhone 3G and this TTY adapter after reading the reviews about this allowing me to use my existing 2.5mm wired headset with my iPhone.
I will report that it does work. However, my voice is so soft that the party on the receiving end of the call can only hear me if they turn there volume all the way up and are in a very quite environment. If I unplug the headset and talk on the phone directly they can hear me loud and clear.
This is using the exact same headset that I used on my previous Motorola flip phone that sounded excellent. So it obviously and adapter/phone issue.
It's a shame that no one feels the need to create a quality wired headset for making phone calls on the iPhone. I could care less about listening to music so a two ear approach is overkill and could be down right dangerous. If I'm driving while using my headset it's important to be able to hear other nosies outside of the context of a phone call. Perhaps that fire truck that's about to blow through the red light and t-bone you if you don't stop on green?
I have yet to find a blue tooth headset that does not sacrifice sound and voice quality for the sake of convenience or to look cool.
Call me old fashioned but the "killer" app on any mobile phone should still be a phone!