T9 dialing was initially introduced as a means of making text input easier on 3x4 numeric keypads, much like those found on old cell phones. Modern smartphones with their phone number hot-linking and advanced contact lists have all but phased out the need for such things, but to those wishing they could access T9 in the iPhone’s native Phone app: you’re in luck.
T9Dialer is a somewhat new jailbreak tweak released by iOS developer Jordan Koch (eskimo_dev) that ports full T9 functionality to the numeric keypad in the iPhone’s native Phone app.