All cell phones made after January of 2004 are required to be TTY compatible. I know for sure that all Sprint cell phones are TTY compatible in which you would hook up an external TTY or VCO device and you can make your calls that way.
There are NO cell phones out there that enable built in TTY functions so that you can make a TTY call by using the keypad of a cell phone. I know that Wyndtell has a special service which you can pay $9.95 a month for TTY calling from your pager to someone with a TTY, but you still use your character limits, so unless you have an unlimited plan, it may cost you if you go over your limit.