Write a function that takes up to four digits of a phone number as a string, and returns a list of all of the words that can be written on the phone with that number. The permuted words should maintain the same order as the input digits. (You should return all permutations, not only English words.) You're provided with a digits-to-letters mapping:
digits_to_letters = { 0: '0', 1: '1', 2: 'ABC', 3: 'DEF', 4: 'GHI', 5: 'JKL', 6: 'MNO', 7: 'PQRS', 8: 'TUV', 9: 'WXYZ' } For example, with the number 27, our function should return a list of all permutations that can be generated using the digits_to_letters mapping: ['AP', 'AQ', 'AR', 'AS', 'BP', 'BQ', 'BR', 'BS', 'CP', 'CQ', 'CR', 'CS']