If the phone number is a real number and the dash has been put in by
formatting (i.e. it does not show in the formula bar), then you can
just change the format to General.
If the dash is incorporated within the phone number (visible in the
formula bar) then you could remove it in one operation using Edit/
Replace. Highlight the column, do CTRL-H, then Find dash, Replace with
nothing, and click Replace All. This will convert the number to a
proper number, so you will lose any leading zeros.
Another way would be by formula in a helper cell:
=SUBSTITUTE(A1,"-","")
and copy this down. This gives a text result, so leading zeros will be
retained.
Hope this helps.
Pete