ADSL uses a different frequency to your phone, hence why you can use both at the same time. These frequencies must be separated:

An inline filter must be used on every device that uses the phone line(e.g. extra phone, fax, eftpos machine, Austar box, etc.).
Like so:
If you do not filter phones and other devices your ADSL will disconnect every time the phone(or other devices) is in use.