[AccessD] Phone Number Input Mask

I'm trying to devise a mask for a 10-digit North-American style phone
number, *plus* room for an optional extension. I modified the built-in mask
but it's not quite working. The mask I have now is:

!\(999") "000\-0000" Ext. "99999;;_

When I enter the number "6477101314234" I expect to see

(647) 710-1314 Ext. 234

and indeed, that's what I'm seeing *until *I press the Tab key to exit the
field. Then the display changes to this:

(  6) 477-1013 Ext. 14234

As BatMan might ask his butler Alfred, "Can you fix my mask?"

TIA,

-- 
Arthur
Cell: 647.710.1314