Spitoon:
Correct - if there's no filter, the signals from the low frequency (voice) bit get mixed with the high-frequency (data) bit. For it to work properly, any analog device (phone, fax, answering machine, dialup modem and the like which previously would have been plugged into a normal socket) needs to be plugged into a microfilter first. The ADSL modem can be plugged straight into the wall (via an RG11 - BT adaptor) if you wish.
Holbrob - you can plug things straight in, but if there's an ADSL modem then voice/fax calls will get clicks and stuff on the line which reduces the quality of the call.