The letter “X” is reserved for dialplan patterns. You should escape it this way :
Almost. The letter X is a reserved *character*; it gets translated internally to [0-9]. So your extension pattern
_FAX_. actually matches the pattern FA[0-9]_. and something like “FAX_3700” is not going to match this pattern (because there is now a letter X in the place where the extension pattern is expecting for there to be a digit 0-9).
Try a backslash before the X?