LetterPhone←{
⍝ ⍵: Character vector of digits e.g. '35'
⍝ ←: All possible letter combinations from a keypad defined by ⍺ 
     ⍺←'_,@' 'ABC' 'DEF' 'GHI' 'JKL' 'MNO' 'PQRS' 'TUV' 'WXYZ'
     ,⊃∘.,/⍺[⍎¨⍵]
 }