Ruby solution to conversion of text & digits into a phone number or back (bidirectional conversion)

kpcnv.rb

def kpcnv(text)
  letters = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"," "]
  numbers = [ 2 , 2,  2 , 3 , 3 , 3 , 4 , 4 , 4 , 5 , 5 , 5 , 6 , 6 , 6 , 7 , 7 , 7 , 7 , 8 , 8 , 8 , 9 , 9 , 9 , 9 , 0 ]
  output = String.new()
  
  for ti in 0..text.length-1
    l = text[ti]
    if l.ord >= 48 and l.ord <= 57 then
      for ni in 0..numbers.length-1
        if numbers[ni] == l.to_i
          output << letters[ni]
          break
        end
      end
    else
      for li in 0..letters.length-1
        if letters[li] == l then
          output << numbers[li].to_s
        end
      end
    end  
  end
  print output
end