Not quite sure what you mean.
If you are trying to use your PC as a FAX while on broadband then that won't work as BB is digital and a FAX is analogue. If you want to use your PC as a FAX machine you need to use a dial up modem.
If you have a Brother standalone FAX machine then it should work.
I think. Actually thinking about it I'm not 100% sure of this, the first part is right you need to use dial up to send faxes and cannot do it on BB(unless you use a company that will reorganise them for you) but the standalone fax machine I ma unsure about, someone else will know though.