Letter Combinations of a Phone Number

Statement

Given a string containing digits from 2 to 9 inclusive, with the possibility of each digit appearing multiple times, return all possible letter combinations that the number could represent. Return the answer in any order.

The illustration below shows the mapping of digits to letters in a telephone dial pad.

Note: The number $1$ on the telephone dial pad does not correspond to any letter, so the input string only contains digits from $2$ to $9$.