Generate all possible words from a number and a phone keyboard

Each key on the telephone represents a list of letters. Given a telephone number, please write a program to output all the possible strings the telephone number represents.

E.g.
phone number has 8 digits
every key has 3 letters
nb of possible strings: 3^8

recursive approach:

// n = list of digits
// get_letters return a list of characters associated to a given digit (actually returns a list of singleton lists)
rec (n)
        if n = []
                return []
        else
                letters = get_letters(n[0])
                res = []
                for l in letters:
                        res.append( 
               l.extend(rec ( right-hand_side (n) ) ) )
                return res