I have a column of numbers in the following format:

X-XX-XXXXXX-X

That is, they're all ten-digit numbers with hyphens in the positions shown.
Or rather, they all SHOULD be like that. Unfortunately, they've been keyed in
by many different people, and sometimes the hyphens have been left out, so
the number looks like this:

XXXXXXXXXX

Is there a way to automate the insertion of the hyphens so I don't have to
do many hundreds of them manually? It would need to be a method I could use
on the column as a whole (and that wouldn't screw up the correct numbers),
because the improperly-formatted numbers are scattered through, and if I have
to select only the incorrect ones to fix, I won't really be saving much time.

Help! Thanks as always!