Can you clarify exactly what you mean by "line number"? Do you mean that you are using Numbering so that every paragraph has a sequential number? If so, use the Numbering option to Define new number format, and set the font there (assuming 2007 is similar to 2010 in this way, as I don't have Word 2007 available here).

I almost always use Multilevel list rather than simple numbering, but the general procedure is the same in either case.