Stuartli and christmascracker are right,you cannot send a fax down a broadband link. Fax is an analogue system, not digital. You can subscribe to an on-line fax system such as efax where you send your fax by E-mail and it is delivered to a fax machine but it is costly with a monthly subscription - or use a good old analogue Fax Modem. A fax modem can be used on the same line as ADSL.
Having said that, with the ubiquitousness of E-mail, fax is all but redundant. The only advantage Fax has over E-mail is that there is an element of proof of transmission and delivery.