Do you have one on an existing phone? You could move that to the fax, and plug the phone into the fax unit. These filters are also commonly available from your favourite electronics or computer supply store, or likely by contacting Telus directly. Search for "ADSL filter" on your favourite electronics supply website.
If your existing phones do not have a filter, it may have been installed at the demarcation point, where the line enters the house. In that instance further filters are not necessary.