phone.rb

def letters_for_number num 
  case num
    when 0, 1
      [num]
    when 2 
      ['A', 'B', 'C'] 
    when 3 
      ['D', 'E', 'F']
    when 4 
      ['G', 'H', 'I']
    when 5 
      ['J', 'K', 'L']
    when 6 
      ['M', 'N', 'O']
    when 7 
      ['P', 'Q', 'R', 'S']
    when 8 
      ['T', 'U', 'V']
    when 9 
      ['W' ,'X', 'Y', 'Z']
    else
      []
  end  
end

def join_sets set, i = 0, str = '', result = []
  return result << str if str.size == set.size
  for item in set[i]    
    join_sets set, i.succ, str + item.to_s, result
  end
  result
end

def phone_options number
  set = number.to_s.split('').map {|digit| letters_for_number digit.to_i}
  join_sets set
end

#test
p phone_options '10-34' #["100DG", "100DH", "100DI", "100EG", "100EH", "100EI", "100FG", "100FH", "100FI"]