Do you still have a number of faxes in the building? Do you still need dedicated Fax (analogue) Lines? How often are they used? Does their usage justify the cost? Could you change the system so that you don’t need them anymore?
By running reports on specific extensions, you can look at the number of incoming or outgoing calls to the fax extensions, you could even monitor the CLI to see if there are specific companies that always use the fax so that you can help them change their ways.