Pay per fax
Maybe you could do a pay per fax option for both outgoing and incoming faxes. Give us a free request for a fax number that is only good for 24, 48, or 72 hours. When we receive the fax, you can tell us who it is from and we can pay .25 cents per page or $1 or $2 for the whole fax. Prices same for outgoing faxes.