Letter Combinations of a Phone Number
Solution overall seems nice, although:


casting to list in list comprehension is not needed
product returns iterator, so you also may want to generate iterator if you mainly plan to iterate over possibilities 
I'd strongly advice using better names. Characters in source code costs nothing and maintaining code afterwards is much easier


My attempt would be:

import itertools

letters_map = {'2':'ABC', '3':'DEF', '4':'GHI', '5':'JKL', 
               '6':'MNO', '7':'PQRS', '8':'TUV', '9':'WXYZ'}

def possible_words(phone_number):
    letters_to_combine = (letters_map[digit] for digit in phone_number)
    for letters_group in itertools.product(*letters_to_combine):
        yield ''.join(letters_group)

print list(possible_words("23567"))