The numbers in the vanity code correspond to letters using the phones number pad. The 1 and 0 do not correspond to any letter and will often be used as spaces. The numbers 2, 3, 4, 5, 6 and 8 are connected to three letters and the 7 and 9 to four letters as shown in the image.
Since one number corresponds to multiple letters, the conversion is not alway unique and more words can be made out of the same number combination (e.g. 34448 can be 'digit' as well as 'eight' or 'fight'). To check which words exist, a dictionary is used. It may happen that words are missing in the result, when these are not included in the used dictionary.