Custom Cell Format for Phone numbers

Hi

Using MS Excel 2003

A single column in spreadsheet contains (and will continue to receive) phone numbers which are:
either 9-digit: 000-000-000
or 10-digit: 000-000-0000

The first digit in either format is a zero, which I want always to display.

Looking for a single Custom format, to format the entire column - which will allow either a 9 digit or a 10 digit string to be entered in any cell and always have the first 2 sets (6 digits, which start with a zero) displaying as 000-000- then allow the final set to show as either 000 or 0000

Is this possible? I've tried all ways I can think of but can't get it to work having a single format which will accept both types of ph.numbers.

I'd like to use the Format>Cells>Custom Format if possible - not VBA or Insert Function.

Thanks
Annieb