User Defined Number Formats for cells - save 4 future use?

I have a custom format of '(###) ###-####' to format telephone numbers. I've named it telephone. But each time I open a new spreadsheet, the UDF format is not there and must be re-created.
How do I permanently save a UDF number format?
Thanks!
Amy
How do I permanently save a UDF number format?
Thanks!
Amy