Voice uses the low frequencies, upstream the high frequencies and downstream the very high frequencies. There are two filtering schemes for sharing the modem with your phone lines. Your phones must be filtered to prevent them from creating or hearing the high frequency clicks used by the modem. The modem has built-in filtering to ignore the low frequency voice.

In old style ADSL, the phone company can install a single splitter and rewire all your phones to go through it to segregate them from the modem. For this option, make sure you have four wires coming to your ADSL jack before you order. The ADSL wall jack looks like two lines, one for phone and one for ADSL. You need two pairs from the central splitter to the jack, one filtered and one unfiltered. The splitter has to be wired so that it goes between the CO and every phone outlet, not just the one where you attach your ADSL.

In new style ADSL, you just use twisted pairst thoughout. You install filter yourself on each phone jack and plug the phones and answeringh machines into the filter. The jack for the ADSL modem does not get a filter. New style is not as fussy about the quality of the wiring.

Your other option is to install a separate filter on every wall jack. You need to install a dual jack for your ADSL, and install a filter in the phone outlet, but not the ADSL outlet.