I'm the one who made the post someone found on the internet, and the tty adapter worked for me. The adapter plugged into the iPhone, and the 2.5mm headset plugged into the adapter. Headset worked fine. Not sure why it wouldn't work for any 2.5mm headset that works on other cell phone.
Only problems were:
No button on the 2.5mm headset...
Adapter sticks out of the top of the iPhone, making it awkward to use.
Weak construction on the adapter itself
Expensive adapter
I used this for a couple of weeks, and then the connector broke when my iPhone was in my pocket. Because it sticks out of the top of the iPhone, it acts like a lever when the headset is attached. As long as you are careful with it, it should work fine. I bent over with my phone in my pocket and my headset attached. The extra length of the adapter was enough to break off when I bent over...
Since it was broken, I did a little disection, and verified that this is simply a 3.5mm to 2.5mm adapter...
I came very close to purchasing another one, and decided to give the existing earbuds a try. I got used to them, and found that they worked pretty well - as long as you can get used to the "in your ear" ear bud feeling.
I ended up getting the bluetooth headset for use in my car. It works very well and for me holds a charge for quite a while. It's kind of pricy, but it works well and is very small. Some people have complained about the interference you can get, but I find that happens mostly when the headset is getting low on power. In the car, it seems to work great - I just put my phone in the cup holder and talk away...
FYI, I also tried some of the "headphone" adapters for listening to music. The Belkin adapter is TERRIBLE. It sticks out over 2" from the top of your phone, and with the heaphones plugged in it is even worse. If you want this type of adapter, I found lots of the original iPod Shuffle headphone adapters on ebay for about $10. They have a 3" iPod headset thin cable going to a female jack for your headphones.
I got the Shure microphone adapter which adds a button and microphone, and it worked OK, but it was weird to talk on the phone with headphones that block out outside sound (it makes your voice sound like your ears are plugged up. You end up with big cable mangement issues too, since you only need about 18" of headphone cord and you usually have 4' +. I didn't care for clipping the mic on my shirt either...
In the end, I decided the stock earbuds worked better for me than I expected for most of my use. When I really want to use my existing headphones, I am going to use the original iPod Shuffle adapter. I like to use closed headphones when I mow the lawn...
