The external filter has two sockets on it, one for phone and the other for ADSL. You should have a filter on each socket, with a phone attached, in the house, although I ran the network at home with at least one of the ext sockets not having a filter and it seemed fine. Not sure if the filter is to protect the quality of the phone or the ADSL side. Probably a bit of both.
I did notice that if we also ran the PCs fax modem into the phone socket on the filter that the fax was a bit unhappy at times. Took a while to connect, send , receive etc. That would be a function of bandwidth limiting through the speech part of the filter.
Anyway, it's worth the wait
ST