phone-number-conversion.vb

Function NormalizeString(Optional ByVal value As Variant) As String
    If value = vbNullString Then
        NormalizeString = ""
    Else
        NormalizeString = StrConv(value, vbUpperCase)
    End If
End Function

Function getPhoneDigit(Optional ByVal value As Variant) As Variant
  getPhoneDigit = NormalizeString(value)

  Select Case value
    Case "A", "B", "C"
      getPhoneDigit = 2
    Case "D", "E", "F"
      getPhoneDigit = 3
    Case "G", "H", "I"
      getPhoneDigit = 4
    Case "J", "K", "L"
      getPhoneDigit = 5
    Case "M", "N", "O"
      getPhoneDigit = 6
    Case "P", "Q", "R", "S"
      getPhoneDigit = 7
    Case "T", "U", "V"
      getPhoneDigit = 8
    Case "W", "X", "Y", "Z"
      getPhoneDigit = 9
  End Select
End Function

Function PhoneNumber(Optional ByVal value As Variant = "") As String
    Dim charArray() As String
    Dim index As Long
    
    value = StrConv(value, vbUnicode)
    value = NormalizeString(value)
    charArray = Split(value, vbNullChar)
       
    For index = LBound(charArray) To UBound(charArray)
        charArray(index) = getPhoneDigit(charArray(index))
    Next index
   
    PhoneNumber = Join(charArray, "")
End Function

Macro-enabled VBA: https://www.dropbox.com/s/nalff660j6n5uxt/phone-number.zip?dl=0

Usage: =PhoneNumber(A2)