Following are the ways ordinary cellphones with only numeric keypads can be used to enter text. See smartphone keyboard
and keyboard types
A standard phone keypad has alphabetic letters on the numeric keys; for example, the "2" is also "ABC." In text mode, the appropriate letters are entered by pressing a key once or several times in rapid succession. With each key press, the next letter in sequence becomes available.
Each physical key has two letters and depressing the key twice in rapid succession enters the second letter.
T9 Numeric Keypad
The T9 method provides major shortcuts when using a numeric keypad (see T9