You can plug any existing analogue device (phone, analogue modem, fax, etc) via a micro fileter, and it will function just like it always has.
The ADSL line is the one which does not need a micro filter, and just plugs straight in to the phone socket. The microfilters exist to keep the ADSL data noises off the regular analogue devices.
If you want to plug in both ADSL and your analogue modem at the same time, you're best to get two cables. Your analogue modem will only rally be necessary as a fall-back if the ADSL is not working (unlikely), or for sending faxes.