update field using substrings of another field

Have a table with fnumber and number, both text
fnumber is a phone number, format "8005551212"
number needs to be the same number but in the format "(800) 555-1212"

we currently run the following each time the list is updated;
update pnums set number='(' || substring(fnumber from 1 for 3) || ') ' ||
substring(fnumber from 4 for 3) || '-' || substring(fnumber from 7 for 4);

is there a way to statically set that number field so that it always equals the
fnumber in that format, or automatically updates itself when the fnumber field
is changed?

thanks

Dave