Duckbutt
In your telephone line install an ADSL filter/splitter near your ADSL modem/router.
There will be two outputs from the splitter/filter, one with ADSL (it will be clearly marked) that you should connect to your ADSL modem/router.
The other will be a phone line output. Use a short piece of telephone cable with suitable plugs/jacks at each end (I think they're RJ4) to connect that to an in-line telephone double adapter. That, in turn, will have two outputs - one to connect to your your phone, the other to your PC's phone line socket on the built in fax modem.
You should then be able to send a fax OR use your phone while still connected to ADSL. You can't do phone and fax together as they're both phone services, but you can do either phone OR fax simultaneously with ADSL on the same line.
Well you can in OZ and we modelled our system on yours, I think.
Cheers
AA