All about numbers in cells here.  Sometimes you need to change certain formatting of cells to fit your needs.

Formating a phone number that begins with a 0

By default Excel removes the 0 infront of the natural numbers (positive integers).  So to format the telephone number you need to tell Excel that the 0 is needed.  To do this right-click and choose Format Cells > Numbers > Custom.  There you need to insert the number of zeros for the positive numbers plus an extra zero for the zero that is missing in the format.  So if the telephone numbers need to start with 0 and consist 10 digits; you need to Type “0000000000”.