What brand of tty (tdd) is comparable with the iPhone? I only have a cell phone. I do not have a landline type phone. Please help. Thanks Much!

I need to get a tty that works with iPhone. I only have this cell phone. I do not have landline phone. What name brands of ttys do I start checking out? I don't want to buy one and then find out it will not work with my iPhone. I know I will need to buy the Apple TTY adapter, but which brands of TTYs go with it? HELP PLEASE. Thanks Much