I have a column of numbers - (telephone numbers actually) and I need
to add a leading zero to each one.

Is there an easy way of doing this other than converting each number
to text and adding the leading zero manually?

You can apply a format to your entire column. Click the column header to
select the entire column, right-click that selection and select Format Cells
from the popup list, then select Custom from the Category list, and type
00-000-000-0000 in the Type field (I am presuming you want your telephone
number to look like I have indicated; otherwise, shape the format string as
you need) and, finally, click OK to finish the process off.

Rick