I need to change the format of phone numbers in excel from the standard 000-000-0000 to #10000000000.

I think if you just go to format-cell and click on the "general" under category the number will appear as 4548675309. Is that what you were looking for?

If your example is EXACTLY what you want, the #1 added to the beginning of a typed in 10 digit telephone number, do this:

Highlight the cells
FORMAT > CELLS > NUMBER > CUSTOM
in the TYPE box, enter: "#1"0000000000
Select FORMAT > CONDITIONAL FORMATTING
Cell Value is / less than / 999999999 (that's nine nines)
Click on FORMAT and set the color to RED
Try it out now. If you enter a perfect 10 digit number it will add the #1 to the beginning. But if you do not enter enough digits, the data will appear in RED and you know you need ot fix it.