Predictive text input is a technology that makes it easier to type text messages. The predictive text input method uses a built-in dictionary to recognise the most commonly used words for each sequence of key presses. This way, you press each key only once, even if the letter you want is not the first letter on the key.
As predictive text input becomes familiar with the words and phrases you are using, it suggests the most frequently used words first. The dictionary can be expanded by adding words, which enables them to be recognised in the future.
To enable and use T9:

