I can't tell from your post where the data originated from, or where it is now.
There are 2 different answers depending on what you're doing... if you are about to enter data into cell, first format the column as Text, and then it will allow you to enter numeric characters and start with 0.
But I suspect you have the fax numbers in Excel, but need to have a zero in from of each one.
If the first fax number is in cell A1, put this in B1:
=0&A1
Copy down.
Now to convert the formula: Select all the formulas and Copy
Use the Paste drop-down and choose Paste Values.