Define the cell format as text.
If you subsequently want to treat it as a number you can use the Value() function to convert it into a number.
e.g where A3 is text '004906783' Value (A3) will give you the number 4906783
It gets much more complicated if you have hyphens and spaces in the telephone number, though actually can't think why you would want to treat one as an actual number anyway.....